Tomori impressed during a loan spell at Milan during the second half of last season.
Now the 23-year-old centre-back is a permanent Rossoneri player, signing for an initial £25 million.
"Wow.. where do I start? A club I've been with since I was 8 years old," Tomori posted on Instagram.
"Growing through all stages of the academy. Helped me develop and grow as a player and a person.
"Winning various trophies throughout all the youth ages, to making my debut at Stamford Bridge, having the chance to play in the Premier League and Champions League; the things I dreamt of as a kid. Memories I will cherish forever.
"All I can say is thank you. To all the staff in the academy that got me to this point, all my teammates, ALL the fans, and Chelsea Football Club. I will miss you all."