Although the fee was not disclosed by the clubs involved, British and US media outlets reported that Chelsea had made an offer worth £97.5 million ($135 million) for Lukaku.
The Belgian joined Chelsea from Anderlecht in August 2011, but left the West London club for Everton in 2014 after making just 15 appearances.
“I’m happy and blessed to be back at this wonderful club,” Lukaku said in a club statement. “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 … the challenge is to try to help the team win some more trophies. I can’t wait to get started and to help the club achieve more success.”
Lukaku was one of the most prolific strikers in the Premier League, scoring 87 goals in 166 games for the Merseyside club between 2014 and 2017.
In July 2017, Lukaku made the move to Manchester United where he added to his goalscoring tally, with 42 goals in 96 appearances.
Lukaku’s stay, though, was short-lived, with the talisman sealing a move to Italian giant Inter Milan in August 2019.
The return of Lukaku to Stamford Bridge will add even more firepower to an attacking line-up which already includes German duo Kai Havertz and Timo Werner, England’s Mason Mount and American Christian Pulisic.
Chelsea director Marina Granovskaia added in the club statement: “Romelu Lukaku is quite simply one of the world’s best strikers and goalscorers. We are absolutely delighted to be bringing him back to the club he loves, and are excited to be adding his talent to our Champions League-winning squad.
“We are of course looking to build on last season’s success, and Lukaku will play a big role in achieving our targets.”
After winning last season’s UEFA Champions League, Thomas Tuchel’s side will have their eyes set on regaining the English Premier League title, which they last won in 2017.
Chelsea begins the new Premier League season at home to Crystal Palace on Saturday.