The London club confirmed the return of the 28-year-old strike for a reported club record fee of £97 million ($135 million) in a statement on Thursday.
“One of the most prolific strikers in European football, Lukaku has returned to the club he supported as a young boy and where he has unfinished business, having been on our books between 2011 and 2014,” the club said.
“The Belgian has signed a five-year contract and will add attacking firepower to Thomas Tuchel’s squad of European champions.”
Lukaku scored 64 goals in two seasons in Italy to quickly become a hero at the San Siro.
But the Belgian also has a proven record in the Premier League, scoring 113 goals during spells with Manchester United, Everton and West Brom.
The only blot on Lukaku’s career so far was his first spell at Chelsea as he failed to score in 15 appearances.
Signed as an 18-year-old as the apparent heir to Didier Drogba, Lukaku quickly grew frustrated at his lack of opportunities in the first team.
Drogba was the hero as Lukaku’s first season as a Chelsea player ended with them winning the Champions League for the first time in the club’s history.
But he later stated he felt he played “no part” in that success due to his limited game time.
“I’m happy and blessed to be back at this wonderful club,” said Lukaku, who has signed a five-year contract. “It’s been a long journey for me: I came here as a kid who had a lot to learn, now I’m coming back with a lot of experience and more mature.
“The way the club is going fits my ambitions perfectly at 28 and just coming off winning Serie A. I think this opportunity comes at the right time and hopefully we can have a lot of success together.”
Prolific loan spells with West Brom and then Everton proved Lukaku’s ability to score goals in the Premier League before he made a permanent move to the Toffees in 2014.
After scoring 87 goals in three seasons with Everton, Chelsea came calling again but Lukaku instead joined Manchester United in an inital £75 million move.