Harvey is graduating from A&M this month and will be eligible to play for the Gamecocks in the fall.
"I can't wait to get to Columbia to see what this season holds!" he said in his Instagram post.
Harvey started 12 games for the Aggies in 2016, and his 10 pass breakups led the team. He also had 68 tackles and an interception.
Harvey missed the 2017 season recovering from a knee injury. Harvey had announced his intention to transfer back in March and said he had narrowed his choices to Arizona, Auburn and Tennessee in addition to South Carolina.
Texas A&M and South Carolina meet in Columbia, South Carolina on Oct. 13.