Filed Pursuant to Rule 424(b)(3)

Registration No. 333-191049

STEADFAST APARTMENT REIT, INC.

SUPPLEMENT NO. 12 DATED DECEMBER 31, 2015

TO THE PROSPECTUS DATED APRIL 15, 2015

This document supplements, and should be read in conjunction with, our prospectus dated April 15, 2015, as supplemented by Supplement No. 10, dated October 15, 2015 and Supplement No. 11, dated November 13, 2015, relating to our offering of up to $1,100,000,000 in shares of our common stock. Terms used and not otherwise defined in this Supplement No. 12 shall have the same meanings as set forth in our prospectus. The purpose of this Supplement No. 12 is to disclose:

• the status of our public offering;

• an update to our suitability standards;

• an update to our subscription agreement; and

• our recent property acquisitions.

Status of Our Public Offering

 We commenced our initial public offering of up to $1,100,000,000 in shares of our common stock on December 30, 2013. As of December 28, 2015, we had received and accepted investors’ subscriptions for and issued 34,833,597 shares of our common stock in our public offering, resulting in gross offering proceeds of $519,772,594. As of December 28, 2015, 32,551,648 shares of our common stock remained available for sale to the public under our initial public offering, excluding shares available under our distribution reinvestment plan.

Our board of directors has extended our initial public offering for up to an additional year through December 30, 2016.  However, we intend to sell shares of our common stock in our initial public offering until on or before March 25, 2016, or the date on which the maximum offering amount has been sold.

Update to Our Suitability Standards

The following information supersedes and replaces the discussion with respect to Kentucky contained in the “Suitability Standards” section of our prospectus beginning on page i.

Kentucky—Kentucky investors may not invest more than 10% of their liquid net worth in us. Moreover, no Kentucky resident shall invest more than 10% of his or her liquid net worth (cash, cash equivalents and readily marketable securities) in our shares or the shares of our affiliate’s non-publicly traded real estate investment trusts.

Update to Our Subscription Agreement

In connection with the revision of our suitability standard for Kentucky investors, we updated our form of subscription agreement. A copy of our updated form of subscription agreement is attached to this supplement as Appendix A.

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Our Recent Property Acquisitions

As described in greater detail below, on December 11, 2015, or the closing date, we acquired PeakView at T-Bone Ranch, or the T-Bone property, a 224-unit multifamily community located in Greeley, Colorado, and Park Valley Apartments, or the Park Valley property, a 496-unit multifamily community located in Smyrna, Georgia. In addition, on December 18, 2015, or the Horseshoe closing date, we acquired PeakView by Horseshoe Lake, or the Horseshoe property, a 222-unit multifamily community located in Loveland, Colorado. Lastly, on December 30, 2015, or the Stoneridge closing date, we acquired Stoneridge Farms, or the Stoneridge property, a 336-unit multifamily community located in Smyrna, Tennessee. With the acquisition of these four properties, we have invested in the aggregate over $1.2 billion in 30 properties in 11 states with 9,858 apartment homes. Our first seven properties were acquired in 2014; and we have purchased 23 properties in 2015.

Acquisition of PeakView at T-Bone Ranch

On the closing date, we acquired from a third-party seller the T-Bone property, through STAR T-Bone, LLC, or STAR T-Bone, a wholly-owned subsidiary of our operating partnership. STAR T-Bone acquired the T-Bone property for an aggregate purchase price of $40,300,000, excluding closing costs. STAR T-Bone financed the payment of the purchase price for the T-Bone property with a combination of (1) proceeds from our ongoing public offering and (2) a loan in the aggregate principal amount of $28,210,000 from Berkeley Point Capital LLC, pursuant to the requirements of the Fannie Mae Multifamily Delegated Underwriting and Servicing Program.

An acquisition fee of approximately $407,000 was earned by our advisor in connection with the acquisition of the T-Bone property. A loan coordination fee of approximately $282,100 was earned by our advisor in connection with the financing of the T-Bone property.

The T-Bone property is a 224-unit residential community constructed on an approximately 16-acre site in 2002. The T-Bone property is comprised of 16 two-story residential buildings, a leasing office and nine detached garage buildings. The apartment homes at the T-Bone property consist of a mix of one, two and three bedroom units averaging 1,025 square feet per unit. Average in-place monthly rent at the T-Bone property was approximately $1,226 as of December 8, 2015. Unit amenities at the T-Bone property include island kitchens, walk-in closets, washer/dryers, fireplaces, track lighting and oversized double pane windows. In addition, select units have nine-foot ceilings, storage closets and patios and balconies. Property amenities at the T-Bone property include a heated pool, hot tub, fitness center, sand volleyball court, business/conference center, dog park, two children’s playgrounds and a clubhouse with lounge. Occupancy at the T-Bone property was approximately 93% as of December 8, 2015. We have engaged Steadfast Management Company, Inc., or SMC, to serve as the property manager and Pacific Coast Land & Construction, Inc., or PCL, to serve as the construction manager for the T-Bone property.

Acquisition of Park Valley Apartments

On the closing date, we acquired from a third-party seller the Park Valley property, through STAR Park Valley, LLC, or STAR Park Valley, a wholly-owned subsidiary of our operating partnership. STAR Park Valley acquired the Park Valley property for an aggregate purchase price of $51,400,000, excluding closing costs. STAR Park Valley financed the payment of the purchase price for the Park Valley property with a combination of (1) proceeds from our ongoing public offering and (2) an advance in the aggregate principal amount of $38,550,000 from PNC Bank, National Association, or the lender, pursuant to our revolving credit facility with the lender.

An acquisition fee of approximately $566,000 was earned by our advisor in connection with the acquisition of the Park Valley property. A loan coordination fee of approximately $385,500 was earned by our advisor in connection with the financing of the Park Valley property.

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The Park Valley property is a 496-unit residential community constructed on an approximately 41-acre site in 1987. The Park Valley property is comprised of 25 two- and three-story residential buildings. The apartment homes at the Park Valley property consist of a mix of studios, one and two bedroom units averaging 781 square feet per unit. Average in-place monthly rent at the Park Valley property was approximately $871 as of December 10, 2015. Unit amenities at the Park Valley property include modern stainless steel/black appliances and granite inspired kitchen and bath countertops. In addition, select units have breakfast bars, full-size washer/dryer connections, large walk-in closets, balconies/patios and screened porches, vaulted ceilings and wood-burning gas fireplaces with mantels. Property amenities at the Park Valley property include a swimming pool, cyber cafe, fitness center, free weekly boot camp, three lighted tennis courts, car care center, picnic areas with grills and a pet park. Occupancy at the Park Valley property was approximately 95% as of December 10, 2015. We have engaged SMC to serve as the property manager and PCL to serve as the construction manager for the Park Valley property.

Acquisition of PeakView by Horseshoe Lake

On the Horseshoe closing date, we acquired from a third-party seller the Horseshoe property, through STAR Horseshoe, LLC, or STAR Horseshoe, a wholly-owned subsidiary of our operating partnership. STAR Horseshoe acquired the Horseshoe property for an aggregate purchase price of $44,200,000, excluding closing costs. STAR Horseshoe financed the payment of the purchase price for the Horseshoe property with a combination of (1) proceeds from our ongoing public offering and (2) an advance in the aggregate principal amount of $33,150,000 pursuant to our revolving credit facility with the lender.

An acquisition fee of approximately $466,000 was earned by our advisor in connection with the acquisition of the Horseshoe property. A loan coordination fee of approximately $331,500 was earned by our advisor in connection with the financing of the Horseshoe property.

The Horseshoe property is a 222-unit residential community constructed on an approximately 14-acre site in 2002. The Horseshoe property is comprised of 13 two- and three-story residential buildings, a leasing office and seven detached garage buildings. The apartments at the Horseshoe property consist of a mix of one, two and three bedroom units averaging 961 square feet per unit. Average in-place monthly rent at the Horseshoe property was approximately $1,338 as of December 15, 2015. Unit amenities at the Horseshoe property include kitchen islands, oversized double pane windows, walk-in closets, track lighting, fireplaces, washers and dryers and microwaves. In addition, select units have a computer nook. Property amenities at the Horseshoe property include a pool, hot tub, fitness center, business center/conference room, dog run, children’s playground, resident clubhouse and a sand volleyball court. Occupancy at the Horseshoe property was approximately 90% as of December 15, 2015. We have engaged SMC to serve as the property manager and PCL to serve as the construction manager for the Horseshoe property.

Acquisition of Stoneridge Farms

On the Stoneridge closing date, we acquired from a third-party seller the Stoneridge property, through STAR Stoneridge, LLC, or STAR Stoneridge, a wholly-owned subsidiary of our operating partnership. STAR Stoneridge acquired the Stoneridge property for an aggregate purchase price of $47,750,000, excluding closing costs. STAR Stoneridge funded the payment of the purchase price for the Stoneridge property with proceeds from our ongoing public offering.

An acquisition fee of approximately $509,000 was earned by our advisor in connection with the acquisition of the Stoneridge property.

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The Stoneridge property is a 336-unit residential community constructed on an approximately 30-acre site in 2005. The Stoneridge property is comprised of 14 three-story residential buildings, a leasing office/clubhouse, a laundry building, pool equipment building and 10 single-story garage buildings. The apartment homes at the Stoneridge property consist of a mix of one, two and three bedroom units averaging 1,162 square feet per unit. Average in-place monthly rent at the Stoneridge property was approximately $1,024 as of December 28, 2015. Unit amenities at the Stoneridge property include a breakfast bar, private sunrooms or patios, fireplaces, walk-in closets and crown molding. In addition, select units include jetted garden tubs and double vanities. Property amenities at the Stoneridge property include a media center, business center, on-site tanning bed, pool and heated spa, playground, lighted tennis court, car care center, fitness center and a pet park. Occupancy at the Stoneridge property was approximately 98% as of December 28, 2015. We have engaged SMC to serve as the property manager and PCL to serve as the construction manager for the Stoneridge property.

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APPENDIX A

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