Midfielder Loftus-Cheek arrives at Milan for a reported fee of around 21 million euros (£18 million) after spending his entire career as a Chelsea player, apart from two loan spells at Crystal Palace and Fulham.
Milan said in a statement that Loftus-Cheek has signed a four-year contract which Italian media report is worth four million euros a season.
The 27-year-old joins fellow England international and Chelsea Academy graduate Fikayo Tomori at the San Siro.
He will replace Italy's Sandro Tonali, who is set to move to Newcastle for around 80 million euros (£68.7 million) as Milan take a new direction without Paolo Maldini.
Milan icon Maldini was sacked from his role as technical director alongside sporting director Frederic Massara earlier this month, a move which angered fans.
Loftus-Cheek has 10 international caps although he hasn't featured for England since 2018.
He will not be the only Chelsea youth product to leave this summer as Mason Mount is set for a big-money move to Manchester United.
