"I'm happy and blessed to be back at this wonderful club," Lukaku said in a 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 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.
"Since I left Chelsea, it's been a long journey with a lot of ups and downs, but these experiences made me strong and 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, 28, scored 47 goals in 72 appearances for Inter since arriving from Manchester United in an €80m deal in 2019.
The striker began his career at Anderlecht before first moving to Chelsea in 2011, where he enjoyed loan spells at West Bromwich Albion and Everton. Lukaku then signed permanently at Everton and later moved to Old Trafford in 2017.
He helped Inter win their first Serie A title in over a decade in the 2020-21 season, where he scored 30 goals across all competitions and was named the league's MVP.
"Romelu Lukaku is quite simply one of the world's best strikers and goalscorers," Chelsea director Marina Granovskaia said. "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."