The Belgium international will be reunited with Jose Mourinho, who signed him for Manchester United back in 2017.
Lukaku spent last season on loan at Inter Milan, helping them win the Coppa Italia and reach the UEFA Champions League final which they lost to Manchester City.
The 30-year-old was not part of new Chelsea manager Mauricio Pochettino's plans for the new season and after failed advances from the Saudi Pro League and Juventus, Lukaku has opted to pitch camp with Roma.
Confirming his exit on Thursday, Chelsea released a blunt statement which read: 'Romelu Lukaku has completed a season-long loan move to Serie A club A.S. Roma.
'The Belgium international, who joined Chelsea in the summer of 2021, spent the previous campaign on loan at Inter Milan.'
'Lukaku will link up in the Italian capital with former Chelsea head coach Jose Mourinho, who guided Roma to the Europa League final last season.'
"The welcome that this club and these fans gave me thrilled me and represents a further incentive to give everything for my new team," Lukaku said.
"I had the opportunity to speak with the owners in recent days and I was struck by their ambition.
"Now we have to work, be humble and grow game after game. For my part, I can't wait to make myself available to my teammates, on and off the field."
Roma's general manager Tiago Pinto added: "Knowing that Lukaku has chosen Roma can only represent a source of great satisfaction for all of us. With his arrival we acquire further experience, professionalism and desire to win".
"All our choices are always aimed at raising Roma's level of competitiveness. Seeing the fans welcome Romelu with that enthusiasm makes us really proud."
Roma have one point from their opening two games of the season and face AC Milan on Friday.
