The 28-year-old Belgium international returns to Stamford Bridge, seven years after leaving the club for Everton in a permanent deal.
Lukaku initially joined Chelsea as a teenager from Anderlecht in the summer of 2011 but made just 15 appearances for the club, spending time on loan at West Bromwich Albion and Everton.
The former Manchester United forward is now Chelsea’s most expensive player.
"I'm happy and blessed to be back at this wonderful club," Lukaku told the club's 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.
"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."
Chelsea director Marina Granovskaia said: "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."
