The 6-foot-8, 240-pound prospect from Nashville, Tennessee, by way of Oak Hill Academy (Virginia), selected the Crimson Tide after also considering Vanderbilt, Texas and Kansas.
Alabama coach Avery Johnson is entering his first season. The Crimson Tide has reached the NCAA tournament just once in the past nine seasons and last won a tournament game in 2006 under Mark Gottfried.
"I have a great relationship with [Johnson]," Key said. "He was a player and coach in the NBA and knows what it takes to get there. I am also very comfortable with assistant coach Bob Simon, who has been recruiting me since I was in ninth grade when he was at Providence."
The four-star Key (formerly known as Braxton Blackwell) was a two-time Class AA Mr. Basketball in the state of Tennessee, averaging 20.7 points, 10.5 rebounds and 4.9 assists in leading Christ Presbyterian Academy to a state title in 2014-15.
He announced his transfer to national high school powerhouse Oak Hill Academy in July.
Key joins Terrance Ferguson (No. 10 in ESPN 100) in the 2016 class for the Crimson Tide, who also will add junior college transfer wing Ar'Mond Davis. Bama is also in the mix for uncommitted center Marques Bolden, No. 20 in the ESPN 100.
"I wanted to do something different," Key said of his decision. "Be a part of something new and special at Alabama with Terrance Ferguson, and now we are going to work on recruiting Marques Bolden and we will be set."