The Giallorossi will pay around £5 million for the striker and cover all his wages.
Lukaku agreed to reduce his salary to allow the move to go through.
He has also agreed to reduce his salary when he returns to Chelsea at the end of the season, and there is a new £37 million release clause in his contract.
Lukaku has finally secured a move away from Chelsea after refusing to be part of their plans for the season.
Chelsea manager Mauricio Pochettino was keen on giving him a second chance at the club, but he was hell-bent on a move away from Stamford Bridge.
Many people thought he’d sign for Inter Milan permanently after his loan spell last season.
However, the 30-year-old considered joining Juventus and Inter pulled out of the negotiations.
Juve never had the funds to sign the Belgian centre-forward and wanted to organise an exchange with Chelsea that would see Dusan Vlahovic move to London.
Chelsea refused the proposal, leaving Lukaku disillusioned. He had an offer from the Saudi Pro League but was not keen on moving to the Middle East.
He has returned to Italy, where he will reunite with Jose Mourinho, who he worked with at Chelsea and Manchester United.
Following the loss of Tammy Abraham to a long-term injury, Roma have been desperately looking for a striker.
Mourinho has now landed Sardar Azmoun and Lukaku as they look to challenge on domestic and European fronts.
Roma will look to bounce back from a poor start to the season when they take on high-flying AC Milan this weekend.
It is unclear if Lukaku will be ready to face the Rossoneri, but he has already started training with his new club.
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