Crimson Tide quarterback Tua Tagovailoa, a junior and possible top-10 pick, will announce his decision Monday.
McKinney, a junior and first-team All-SEC selection, announced the decision on his Instagram page Sunday afternoon.
ESPN NFL draft expert Mel Kiper Jr. lists McKinney as the second-highest-rated draft-eligible safety.
McKinney led the team in tackles this season with 95. He also had three sacks, three interceptions, four forced fumbles and five pass breakups.
A 6-foot-1 native of Georgia, McKinney played primarily on special teams as a freshman before starting the past two seasons at Alabama.
The loss of McKinney means Alabama will be without four of its five starters in the secondary. Safety Jared Mayden, nickelback Shyheim Carter and cornerback Trevon Diggs decided to enter the NFL draft as seniors.