Shevchenko has been repeatedly linked with a move back to Milan
Ukraine international Andriy Shevchenko has completed his move to AC Milan from Chelsea after passing a medical.
The 31-year-old spent two seasons in west London after signing from Milan for about £30m in 2006, but has failed to make an impact at Stamford Bridge.
"I'm so very happy to be here," he told the Italian club's official website.
"At this special moment, I want to send a message to the Chelsea fans who have always made me feel their support. A heartfelt thank you for that."
Shevchenko, AC Milan's second-most prolific goalscorer of all-time, with 127 goals from 208 games, was unable to hold down a regular spot at Chelsea, scoring only nine goals in his 47 Premier League appearances, but maintains he enjoyed his time at the club.
"It was unfortunate I suffered a number of injuries during my time here but I always gave my all, whatever the circumstances", he said.
"I really have enjoyed playing in England and have made many friendships that will stay with me forever.
"I will always look back with fond memories of my time at this really great club, and I would also like to express my gratitude to Roman Abramovich for all his help and encouragement."
Despite his lack of success at Chelsea, Shevchenko has always been held in high regard by Milan's fans and staff.
Milan vice-president Adriano Galliani has expressed his joy at seeing the striker back at the San Siro and insists his return was not solely down to the club's owner Silvio Berlusconi.
Andriy made a mistake in leaving and I think after two years he has understood that this is his home
Milan chief Adriano Galliani
"This is a family and we're all happy," he said.
"It's not true that only Silvio Berlusconi insisted that Andriy return. I've put my weight behind the cause too and I'm happy that in the end we've succeeded in bringing him home."
He added: "After a series of frenetic phone calls during the whole day, we reached an agreement with Chelsea.
"We have managed to bring home a player who has scored the most goals in our shirt in the last 50 years."
Shevchenko's first season at Stamford Bridge ended prematurely when he had hernia surgery, forcing the striker to miss the Champions League semi-final against Liverpool and the FA Cup final.
New Chelsea coach Luiz Felipe Scolari did not pick him for their first Premier League match of the campaign last weekend, prompting Milan to launch the latest in a string of attempts to re-sign the player.
AS Roma and Sampdoria also expressed an interest but returning to Milan was always Shevchenko's preferred choice.
"For me, it's like winning the Champions League," Shevchenko told Italy's ANSA news agency. "There were complications but now everything is resolved I'm really happy."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.