The England international has signed a five-year deal after completing his switch to Goodison Park on 3 July. Keane becomes the fifth signing of this summer transfer window for the Merseyside club, following Jordan Pickford, Davy Klaassen, Henry Onyekuru and Sandro Ramirez.
The 24-year-old had one year left on his contract at Turf Moor and his impressive form has seen him make his senior debut for England earlier in March. Also, his display for Sean Dyche's side saw him attract interest from the top clubs in the Premier League.
Manchester United were the front-runners in re-signing their former player. However, Jose Mourinho decided against signing Keane and secured the services of Swedish international Victor Lindelof from Benfica.
Despite the arrival of Lindelof, the Red Devils wanted to bring Keane back to Old Trafford. However, the centre-back rejected the approach from his former club due to concerns over playing time under the Portuguese tactician.
Keane revealed that having Ronald Koeman in charge at Everton was one of the "big factors" in his decision to join the Toffees. The defender believes the club's style of play will suit him and stressed how the Dutchman convinced him to join him at Goodison Park.
"I'm absolutely delighted to be at Everton, a fantastic football club with passionate fans and a great tradition and I can't wait to come into training tomorrow to meet all the lads," Keane told Everton's official website.
"First and foremost, the manager was a big factor in me coming to the Club. He played in my position when he was a world-class player for a great Barcelona side and a great Dutch side and the style of football he likes to play will suit me, I believe.
"I watched a lot of Everton last season and I believe I really suit how the team likes to play. I feel like this is a great place to come and continue my development as a player.
"He's convinced me that he can still improve me in certain areas of my game. I'm only 24 which, for a centre-half in the Premier League, is relatively young. I've still got a long way to go and I know this and hopefully under his guidance I'll keep improving.
"You always want to play for a manager who believes in you. I wanted to come somewhere I was wanted by the staff, players and fans as well. I feel like Everton is the perfect match.