"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.

[7] Hayes continued promoting the single by posting twenty-one consecutive tweets containing only solid-black images, alluding to the song's title.

[9][10] On May 21, 2015, the song was released to streaming services and to iTunes UK, and was made available to country radio.

[13] "21" debuted at number 32 on the Billboard Country Airplay chart for the week ending June 6, 2015.