Emerson, 25, made 101 appearances for Spurs after joining the club from Barcelona in August 2021 for £25.8m. He marked a century of club appearances against Liverpool at the end of last season.
The Brazilian international, who has 10 caps for his country, joins Milan after the Serie A side saw off interest from Borussia Dortmund and clubs in Saudi Arabia. An initial £8.4m bid from Milan was rejected by Spurs last month, but the Italian giants ultimately secured the defender's signature.