"21" is a song recorded by American country music singer Hunter Hayes as the lead single from his third studio album The 21 Project (2015). Written by Dallas Davidson, Kelley Lovelace, and Ashley Gorley, the song is Hayes' first to not feature an official writing credit. It was released in the UK on May 21, 2015 before being released to digital retailers in North America on May 26, 2015. It is also included on his second extended play, 21 (2015). "21" impacted country radio on June 8, 2015 and entered the Billboard Country Airplay chart at number 32.

21 is the second extended play by American country music artist Hunter Hayes, released to streaming services on August 7, 2015. It is expected to be made available to digital retailers at a later date and serves as a prelude to Hayes's forthcoming third studio album, scheduled for released in the fall of 2015. 21 includes all of the digital-only singles releases from Hayes Spotify-exclusive "The Drop" campaign, led by the project's eponymous lead single, "21", released on May 21, 2015.