"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.

Hunter Easton Hayes is an American multi-genre singer, songwriter, record producer, and multi-instrumentalist. He is proficient at more than 30 instruments.