The 28-year-old is making a return to Stamford Bridge after leaving in 2014.
He has revealed the reasons behind his return back to Chelsea.
“It has 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,” Lukaku told the Chelsea website.
“The way the club is going fits my ambitions perfectly at 28 and just coming off winning Serie A,” Lukaku continued.
“I think this opportunity comes at the right time and hopefully we can have a lot of success together.
Lukaku is in line to make his Chelsea debut on Saturday when they welcome Crystal Palace to Stamford Bridge.