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Ten years after he joined as an 18-year-old from Anderlecht, the Belgian striker has returned to West London after firing Inter Milan to the Scudetto last term.
Lukaku has signed a five-year contract and will be hoping to achieve league title success with the Blues, who won the Champions League under Thomas Tuchel.
Chelsea have paid a club-record fee to bring back Lukaku and they have also made him the highest-paid player at the club by a long way.
As per The Athletic, Lukaku is in line to earn between £325,000 and £450,000-per-week when image rights and performance-related bonuses are taken into account.
Prior to Lukaku's arrival, midfielder N'Golo Kante had been at the top of the club's wage bill following a £290,000-a-week deal he signed in 2018.
With the bonuses factored in, Lukaku could well become the highest-paid player in the Premier League - usurping his compatriot Kevin De Bruyne.
Lukaku has also become the most expensive player in history in terms of total transfer fees. His series of moves have cost just short of £300 million, which is more than Neymar, Cristiano Ronaldo and Alvaro Morata.
In his first spell in West London, Lukaku struggled to get going and didn't score in 15 appearances before being loaned out to West Brom and Everton, who he then joined for £28 million in 2014 after a superb season.
Now though Lukaku feels more mature and complete and ready to bring the goals Chelsea need.
"I'm happy and blessed to be back at this wonderful club," Lukaku told the official Chelsea website.
"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 relationship I have with this club means so much to me, as you know. I have supported Chelsea as a kid and now to be back and try to help them win more titles is an amazing feeling.
"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."
Chelsea haven't yet announced Lukaku's shirt number but a leak from the Premier League's official website suggests he will be wearing the same No.18 jersey he wore in his last spell.