Grier, 15, grew in popularity online originally with his brother Nash and then on his own, according to his website. Hayes now has 4 million followers on Vine.
He'll join Bindi Irwin, Victor Espinoza and Nick Carter in the competition, along with other yet-to-be revealed stars.
Irwin, the 17-year-old daughter of "Crocodile Hunter" Steve Irwin, was the first to officially join the cast.
Victor Espinoza, the jockey who won the Triple Crown in 2015 while riding on the horse American Pharoah, will also be dancing. Prior to Espinoza's win, the last time the Triple Crown feat had been accomplished was in 1978.
Entertainer Nick Carter, 35, is best known for being a member of the Backstreet Boys.
ABC has also announced many of the professional dancers that will join the show this season, and fans will recognize some familiar faces.
PHOTOS: Dancing with the Stars season 21 professional dancers
The celebrity cast announcements will continue all week long.
Dancing with the Stars season 21 will premiere Sept. 14 on ABC at 8 p.m. ET | 7 p.m. CT.