Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.     ☒ Yes    ☐ No

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).     ☒ Yes    ☐ No

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company.  See the definitions of "large accelerated filer," "accelerated filer" and "smaller reporting company" in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).     ☐ Yes    ☒ No

(1)  The condensed statements included herein have been prepared by the registrant, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission, and reflect all adjustments which are, in the opinion of management, necessary to a fair statement of the results of operations for the interim period, on a basis consistent with the annual audited statements.  The adjustments made to these condensed statements consist only of normal recurring adjustments.  Certain information, accounting policies, and footnote disclosures normally included in financial statements prepared in accordance with United States Generally Accepted Accounting Principles (US GAAP) have been condensed or omitted pursuant to such rules and regulations, although the registrant believes that the disclosures are adequate to make the information presented not misleading.  It is suggested that these condensed financial statements be read in conjunction with the financial statements and the summary of significant accounting policies and notes thereto included in the registrant's latest annual report on Form 10‑K.

AEI Income & Growth Fund XXI Limited Partnership ("Partnership") was formed to acquire and lease commercial properties to operating tenants.  The Partnership's operations are managed by AEI Fund Management XXI, Inc. ("AFM"), the Managing General Partner.  Robert P. Johnson, the President and sole director of AFM, serves as the Individual General Partner.  AFM is a wholly owned subsidiary of AEI Capital Corporation of which Mr. Johnson is the majority shareholder.  AEI Fund Management, Inc. ("AEI"), an affiliate of AFM, performs the administrative and operating functions for the Partnership.

The terms of the Partnership offering called for a subscription price of $1,000 per Limited Partnership Unit, payable on acceptance of the offer.  The Partnership commenced operations on April 14, 1995 when minimum subscriptions of 1,500 Limited Partnership Units ($1,500,000) were accepted.  On January 31, 1997, the offering terminated when the maximum subscription limit of 24,000 Limited Partnership Units was reached.  Under the terms of the Limited Partnership Agreement, the Limited Partners and General Partners contributed funds of $24,000,000 and $1,000, respectively.

During operations, any Net Cash Flow, as defined, which the General Partners determine to distribute will be distributed 90% to the Limited Partners and 10% to the General Partners; provided, however, that such distributions to the General Partners will be subordinated to the Limited Partners first receiving an annual, noncumulative distribution of Net Cash Flow equal to 10% of their Adjusted Capital Contribution, as defined, and, provided further, that in no event will the General Partners receive less than 1% of such Net Cash Flow per annum.  Distributions to Limited Partners will be made pro rata by Units.

Any Net Proceeds of Sale, as defined, from the sale or financing of properties which the General Partners determine to distribute will, after provisions for debts and reserves, be paid in the following manner: (i) first, 99% to the Limited Partners and 1% to the General Partners until the Limited Partners receive an amount equal to: (a) their Adjusted Capital Contribution plus (b) an amount equal to 10% of their Adjusted Capital Contribution per annum, cumulative but not compounded, to the extent not previously distributed from Net Cash Flow;  (ii) any remaining balance will be distributed 90% to the Limited Partners and 10% to the General Partners.  Distributions to the Limited Partners will be made pro rata by Units.

For tax purposes, profits from operations, other than profits attributable to the sale, exchange, financing, refinancing or other disposition of property, will be allocated first in the same ratio in which, and to the extent, Net Cash Flow is distributed to the Partners for such year.  Any additional profits will be allocated in the same ratio as the last dollar of Net Cash Flow is distributed.  Net losses from operations will be allocated 99% to the Limited Partners and 1% to the General Partners.

For tax purposes, profits arising from the sale, financing, or other disposition of property will be allocated in accordance with the Partnership Agreement as follows: (i) first, to those partners with deficit balances in their capital accounts in an amount equal to the sum of such deficit balances; (ii) second, 99% to the Limited Partners and 1% to the General Partners until the aggregate balance in the Limited Partners' capital accounts equals the sum of the Limited Partners' Adjusted Capital Contributions plus an amount equal to 10% of their Adjusted Capital Contributions per annum, cumulative but not compounded, to the extent not previously allocated; (iii) third, the balance of any remaining gain will then be allocated 90% to the Limited Partners and 10% to the General Partners.  Losses will be allocated 98% to the Limited Partners and 2% to the General Partners.

The General Partners are not required to currently fund a deficit capital balance.  Upon liquidation of the Partnership or withdrawal by a General Partner, the General Partners will contribute to the Partnership an amount equal to the lesser of the deficit balances in their capital accounts or 1% of total Limited Partners' and General Partners' capital contributions.

In January 2014, the Managing General Partner mailed a Consent Statement (Proxy) seeking the consent of the Limited Partners to continue the Partnership for an additional 60 months or to initiate the final disposition, liquidation and distribution of all of the Partnership's properties and assets.  On February 14, 2014, the proposal to continue the Partnership was approved with a majority of Units voted in favor of the continuation proposal.  As a result, the Managing General Partner will continue the operations of the Partnership for an additional 60 months at which time it will again ask the Limited Partners to vote on the same two proposals.

On February 3, 2016, the Partnership purchased a Dollar Tree store in Cincinnati, Ohio for $1,809,915.  The Partnership allocated $285,049 of the purchase price to Acquired Intangible Lease Assets, representing in-place lease intangibles, and allocated $80,404 to Acquired Below-Market Lease Intangibles. The Partnership incurred $56,760 of acquisition expenses related to the purchase that were expensed.  The property is leased to Dollar Tree Stores, Inc. under a Lease Agreementlease agreement with a remaining primary term of 10 years (as of the date of purchase) and annual rent of $122,169.

ForThe Partnership owns a 30% interest in the nineGander Mountain store in Champaign, Illinois.  The remaining interests in the property are owned by affiliates of the Partnership.  On March 10, 2017, Gander Mountain Company filed for Chapter 11 reorganization and announced it was closing the store, following a liquidation sale of its on‑site assets.  In June 2017, the tenant filed a motion with the bankruptcy court to reject the lease for this store effective June 30, 2017.  At this time, the tenant returned possession of the property to the owners and the Partnership became responsible for its 30% share of real estate taxes and other costs associated with maintaining the property.  The tenant paid rent through June 2017.  The owners have listed the property for lease with a real estate broker in the Champaign area.  The annual rent from this property represented approximately 13% of the total annual rent of the Partnership's property portfolio.  The loss of rent and increased expenses related to this property decreased the Partnership's cash flow.  Consequently, beginning with the third quarter of 2017, the Partnership reduced its regular quarterly cash distribution rate from $13.18 per Unit to $10.51 per Unit.

In March 2017, the Partnership entered into an agreement with the tenant of the KinderCare daycare center in Andover, Minnesota to extend the lease term five years to expire on June 30, 2022.  The annual rent will remain the same throughout the remainder of the extended lease term.  As part of the agreement, the Partnership paid a tenant improvement allowance of $30,000 that was capitalized.  In addition, beginning on April 1, 2017, the tenant received free rent for three months endedthat equaled $36,362.  In the first quarter of 2017, the Partnership decided to sell the property.  At September 30, 2016 and 2015,2017, the property was classified as Real Estate Held for Sale with a carrying value of in-place lease intangibles amortized$652,572.

In October 2017, the Partnership entered into an agreement to expense was $52,783sell the KinderCare to an unrelated third party.  The sale is subject to contingencies and $36,015 andmay not be completed.  If the increasesale is completed, the Partnership expects to rental income for below-market leases was $12,974 and $8,244, respectively.  For lease intangibles not held for sale asreceive net proceeds of September 30, 2016,approximately $1,695,000, which will result in a net gain of approximately $1,042,400.

In April 2017, the weighted average remaining life is 117 months for in-place lease intangibles and 86 months for below-market leases.  The estimated amortization expense is $76,764 andPartnership entered into an agreement with the estimated increase to rental income is $19,100 for eachtenant of the next five succeeding years.Fresenius Medical Center in Shreveport, Louisiana to extend the lease term nine years to expire on June 30, 2027.  The annual rent will remain the same throughout the remainder of the extended lease term.  As part of the agreement, the Partnership paid a tenant improvement allowance of $46,750 that was capitalized and will be depreciated.

AEI Fund Management, Inc. performs the administrative and operating functions for the Partnership.  The payable to AEI Fund Management represents the balance due for those services.  This balance is non-interest bearing and unsecured and is to be paid in the normal course of business.

For the nine months ended September 30, 20162017 and 2015,2016, the Partnership declared distributions of $786,569$730,702 and $822,818,$786,569, respectively. The Limited Partners received distributions of $778,703$723,395 and $814,590$778,703 and the General Partners received distributions of $7,866$7,307 and $8,228$7,866 for the periods, respectively.  The Limited Partners' distributions represented $39.07$36.87 and $38.64$39.07 per Limited Partnership Unit outstanding using 19,92919,622 and 21,08319,929 weighted average Units in 20162017 and 2015,2016, respectively.  The distributions represented $17.87$2.41 and $16.54$17.87 per Unit of Net Income and $21.20$34.46 and $22.10$21.20 per Unit of return of contributed capital in 20162017 and 2015,2016, respectively.

As part of the distributions discussed above, the Partnership distributed net sale proceeds (from property sales completed in 2014) of $139,022$68,812 in 2015.2017.  The Limited Partners received distributions of $137,632$68,124 and the General Partners received distributions of $1,390.$688.  The Limited Partners' distributions represented $6.51$3.47 per Unit.

As of September 30, 20162017 and December 31, 2015,2016, the Partnership had no assets or liabilities measured at fair value on a recurring basis or nonrecurring basis.

This section contains "forward-looking statements" which represent management's expectations or beliefs concerning future events, including statements regarding anticipated application of cash, expected returns from rental income, growth in revenue, the sufficiency of cash to meet operating expenses, rates of distribution, and other matters.  These, and other forward-looking statements, should be evaluated in the context of a number of factors that may affect the Partnership's financial condition and results of operations, including the following:

The Partnership's financial statements have been prepared in accordance with US GAAP.  Preparing the financial statements requires management to use judgment in the application of these accounting policies, including making estimates and assumptions.  These judgments will affect the reported amounts of the Partnership's assets and liabilities and the disclosure of contingent assets and liabilities as of the dates of the financial statements and will affect the reported amounts of revenue and expenses during the reporting periods.  It is possible that the carrying amount of the Partnership's assets and liabilities, or the results of reported operations, will be affected if management's estimates or assumptions prove inaccurate.

Management of the Partnership evaluates the following accounting estimates on an ongoing basis, and has discussed the development and selection of these estimates and the management discussion and analysis disclosures regarding them with the managing partner of the Partnership.

Upon acquisition of real properties, the Partnership records them in the financial statements at cost.  The purchase price is allocated to tangible assets, consisting of land and building, and to identified intangible assets and liabilities, which may include the value of above market and below market leases and the value of in-place leases.  The allocation of the purchase price is based upon the fair value of each component of the property.  Although independent appraisals may be used to assist in the determination of fair value, in many cases these values will be based upon management's assessment of each property, the selling prices of comparable properties and the discounted value of cash flows from the asset.

The fair values of above market and below market in-place leases will be recorded based on the present value (using an interest rate which reflects the risks associated with the leases acquired) of the difference between (i) the contractual amounts to be paid pursuant to the in-place leases and (ii) an estimate of fair market lease rates for the corresponding in-place leases measured over a period equal to the non-cancelable term of the lease including any bargain renewal periods.  The above market and below market lease values will be capitalized as intangible lease assets or liabilities.  Above market lease values will be amortized as an adjustment of rental income over the remaining termsterm of the respective leases.  Below market leaseslease values will be amortized as an adjustment of rental income over the remaining term of the respective leases, including any bargain renewal periods.  If a lease were to be terminated prior to its stated expiration, all unamortized amounts of above market and below market in-place lease values relating to that lease would be recorded as an adjustment to rental income.

The fair values of in-place leases will include estimated direct costs associated with obtaining a new tenant, and opportunity costs associated with lost rentals which are avoided by acquiring an in-place lease.  Direct costs associated with obtaining a new tenant may include commissions, tenant improvements, and other direct costs and are estimated, in part, by management's consideration of current market costs to execute a similar lease.  These direct costs will be included in intangible lease assets on the balance sheet and will be amortized to expense over the remaining term of the respective leases.  The value of opportunity costs will be calculated using the contractual amounts to be paid pursuant to the in-place leases over a market absorption period for a similar lease.  These intangibles will be included in intangible lease assets on the balance sheet and will be amortized to expense over the remaining term of the respective leases.  If a lease were to be terminated prior to its stated expiration, all unamortized amounts of in-place lease assets relating to that lease would be expensed.

The determination of the fair values of the assets and liabilities acquired will require the use of significant assumptions with regard to the current market rental rates, rental growth rates, discount and capitalization rates, interest rates and other variables.  If management's estimates or assumptions prove inaccurate, the result would be an inaccurate allocation of purchase price, which could impact the amount of reported net income.

Properties are carried at original cost, less accumulated depreciation and amortization.  The Partnership tests long-lived assets for recoverability when events or changes in circumstances indicate that the carrying value may not be recoverable.  For properties the Partnership will hold and operate, management determines whether impairment has occurred by comparing the property's probability-weighted future undiscounted cash flows to its current carrying value.  For properties held for sale, management determines whether impairment has occurred by comparing the property's estimated fair value less cost to sell to its current carrying value.  If the carrying value is greater than the net realizable value, an impairment loss is recorded to reduce the carrying value of the property to its net realizable value.  Changes in these assumptions or analysis may cause material changes in the carrying value of the properties.

AEI Fund Management, Inc. allocates expenses to each of the funds they manage primarily on the basis of the number of hours devoted by their employees to each fund's affairs.  They also allocate expenses at the end of each month that are not directly related to a fund's operations based upon the number of investors in the fund and the fund's capitalization relative to other funds they manage.  The Partnership reimburses these expenses subject to detailed limitations contained in the Partnership Agreement.

For the nine months ended September 30, 20162017 and 2015,2016, the Partnership recognized rental income of $971,456$924,167 and $884,694,$971,456, respectively.  In 2016,2017, rental income increaseddecreased due to the tenant of the KinderCare daycare center receiving free rent, and the Gander Mountain situation, as discussed below.  These decreases were partially offset by additional rent received from one property acquisition in 2016 and a rent increaseincreases on one property.two properties.  Based on the scheduled rent for the properties as of October 31, 2016,2017, the Partnership expects to recognize rental income of approximately $1,306,000$1,216,000 and $1,264,000$1,182,000 in 20162017 and 2017,2018, respectively.

For the nine months ended September 30, 20162017 and 2015,2016, the Partnership incurred Partnership administration expenses from affiliated parties of $127,367$128,126 and $165,505,$127,367, respectively.  These administration expenses include costs associated with the management of the properties, processing distributions, reporting requirements and communicationcommunicating with the Limited Partners.  During the same periods, the Partnership incurred Partnership administration and property management expenses from unrelated parties of $40,075$88,753 and $35,486,$40,075, respectively.  These expenses represent direct payments to third parties for legal and filing fees, direct administrative costs, outside audit costs, taxes, insurance and other property costs.  These expenses were higher in 2017, when compared to 2016, due to expenses related to the Gander Mountain store.

For the nine months ended September 30, 2016, the Partnership incurred property acquisition expenses of $56,760 related to the purchase of the Dollar Tree store in Cincinnati, Ohio.

For the nine months ended September 30, 20162017 and 2015,2016, the Partnership recognized interest income of $2,361$1,097 and $8,337,$2,361, respectively.

Management believes inflation has not significantly affected income from operations.  Leases may contain rent increases, based on the increase in the Consumer Price Index over a specified period, which will result in an increase in rental income over the term of the leases.  Inflation also may cause the real estate to appreciate in value.  However, inflation and changing prices may have an adverse impact on the operating margins of the properties' tenants, which could impair their ability to pay rent and subsequently reduce the Net Cash Flow available for distributions.

During the nine months ended September 30, 2017, the Partnership's cash balances decreased $188,630 as a result of cash paid for tenant improvement allowances, and distributions paid to the Partners and cash used to repurchase Units in excess of cash generated from operating activities.  During the nine months ended September 30, 2016, the Partnership's cash balances decreased $2,068,977 as a result of cash used to purchase property, and distributions paid to the Partners and cash used to repurchase Units in excess of cash generated from operating activities.  During the nine months ended September 30, 2015, the Partnership's cash balances decreased $1,059,814 as a result of distributions paid to the Partners and cash used to repurchase Units in excess of cash generated from operating activities.

Net cash provided by operating activities increaseddecreased from $696,049 in 2015 to $765,870 in 2016 to $689,546 in 2017 as a result of an increasea decrease in total rental and interest income in 2016, a decrease2017, an increase in Partnership administration and property management expenses in 2016,2017 and net timing differences in the collection of payments from the tenants and the payment of expenses.  During 2016, cash from operations was reduced by $56,760 of acquisition expenses related to the purchase of real estate.  Pursuant to accounting guidance, these expenses were reflected as operating cash outflows.  However, pursuant to the Partnership Agreement, acquisition expenses were funded with proceeds from property sales.

The major components of the Partnership's cash flow from investing activities are investments in real estate and proceeds from the sale of real estate.  During the nine months ended September 30, 2017 and 2016, the Partnership expended $76,750 and $1,809,915, respectively, to invest in real properties as the Partnership reinvested cash generated from property sales completed in 2014.

On February 3, 2016, the Partnership purchased a Dollar Tree store in Cincinnati, Ohio for $1,809,915.  The property is leased to Dollar Tree Stores, Inc. under a Lease Agreementlease agreement with a remaining primary term of 10 years (as of the date of purchase) and annual rent of $122,169.

The Partnership's primary use of cash flow, other than investment in real estate, is distribution payments to Partners and cash used to repurchase Units.  The Partnership declares its regular quarterly distributions before the end of each quarter and pays the distribution in the first week after the end of each quarter.  The Partnership attempts to maintain a stable distribution rate from quarter to quarter.  The Partnership may repurchase tendered Units on April 1st and October 1st of each year subject to limitations.

For the nine months ended September 30, 20162017 and 2015,2016, the Partnership declared distributions of $786,569$730,702 and $822,818,$786,569, respectively, which were distributed 99% to the Limited Partners and 1% to the General Partners.  The Limited Partners received distributions of $778,703$723,395 and $814,590$778,703 and the General Partners received distributions of $7,866$7,307 and $8,228$7,866 for the periods, respectively.

As part of the distributions discussed above, the Partnership distributed net sale proceeds (from property sales completed in 2014) of $139,022$68,812 in 2015.2017.  The Limited Partners received distributions of $137,632$68,124 and the General Partners received distributions of $1,390.$688.  The Limited Partners' distributions represented $6.51$3.47 per Unit.

The Partnership may repurchase Units from Limited Partners who have tendered their Units to the Partnership.  Such Units may be acquired at a discount.  The Partnership will not be obligated to purchase in any year more than 5% of the total number of Units outstanding on January 1 of such year.  In no event shall the Partnership be obligated to purchase Units if, in the sole discretion of the Managing General Partner, such purchase would impair the capital or operation of the Partnership.

On April 1, 2017, the Partnership repurchased a total of 20.00 Units for $17,616 from one Limited Partner in accordance with the Partnership Agreement.  The Partnership acquired these Units using Net Cash Flow from operations.  On April 1, 2016, the Partnership repurchased a total of 264.01 Units for $233,076 from 12 Limited Partners in accordance with the Partnership Agreement.  On April 1, 2015, the Partnership repurchased a total of 1,118.25 Units for $902,509 from 27 Limited Partners. The Partnership acquired these Units using net sale proceeds.  The repurchases increase the remaining Limited Partners' ownership interest in the Partnership.  As a result of these repurchases and pursuant to the Partnership Agreement, the General Partners received distributions of $178 and $2,355 in 2017 and $9,117 in 2016, and 2015, respectively.

The continuing rent payments from the properties, together with cash generated from property sales, should be adequate to fund continuing distributions and meet other Partnership obligations on both a short-term and long-term basis.

As of September 30, 20162017 and December 31, 2015,2016, the Partnership had no material off-balance sheet arrangements that had or are reasonably likely to have current or future effects on its financial condition, results of operations, liquidity or capital resources.

Not required for a smaller reporting company.

(a)  Disclosure Controls and Procedures.

Under the supervision and with the participation of management, including its President and Chief Financial Officer, the Managing General Partner of the Partnership evaluated the effectiveness of the design and operation of our disclosure controls and procedures (as defined in Rule 13a-15(e) under the Securities Exchange Act of 1934 (the "Exchange Act")).  Based upon that evaluation, the President and Chief Financial Officer of the Managing General Partner concluded that, as of the end of the period covered by this report, our disclosure controls and procedures were effective in ensuring that information required to be disclosed by us in the reports that we file or submit under the Exchange Act is recorded, processed, summarized and reported within the time periods specified in applicable rules and forms and that such information is accumulated and communicated to management, including the President and Chief Financial Officer of the Managing General Partner, in a manner that allows timely decisions regarding required disclosure.

(b)  Changes in Internal Control Over Financial Reporting.

During the most recent period covered by this report, there has been no change in our internal control over financial reporting (as defined in Rule 13a-15(f) under the Exchange Act) that has materially affected, or is reasonably likely to materially affect, our internal control over financial reporting.

There are no material pending legal proceedings to which the Partnership is a party or of which the Partnership's property is subject.

Not required for a smaller reporting company.

(a) None.

(b) Not applicable.

(c) Pursuant to Section 7.7 of the Partnership Agreement, as amended, each Limited Partner has the right to present Units to the Partnership for purchase by submitting notice to the Managing General Partner during January or July of each year.  The purchase price of the Units is equal to 95% of the net asset value per Unit, as of the first business day of January or July of each year, as determined by the Managing General Partner in accordance with the provisions of the Partnership Agreement.  Units tendered to the Partnership during January and July may be repurchased on April 1^st and October 1^st, respectively, of each year subject to the following limitations.  The Partnership will not be obligated to purchase in any year more than 5% of the total number of Units outstanding on January 1 of such year.  In no event shall the Partnership be obligated to purchase Units if, in the sole discretion of the Managing General Partner, such purchase would impair the capital or operation of the Partnership.  During the period covered by this report, the Partnership did not purchase any Units.

None.

Not Applicable.

None.

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.