The former Inter Milan superstar is the world-class striker Blues boss Thomas Tuchel has been hunting all summer.
The 28 year old Belgium hero returns to Stamford Bridge seven years after leaving for Everton, Manchester United and then the Serie A giants.
Before clinching the second-biggest signing by a British club, Tuchel explained why he rates Lukaku so highly.
Tuchel said: “He is the kind of profile we’re looking for…
“Romelu is one of the guys like Erling Haaland at Dortmund, Robert Lewandowski at Bayern Munich, Harry Kane at Tottenham, who is a real No 9, who loves to score and who has a presence in the box.
“That is not a secret. The sentence is easy to repeat for any coach in the world. We lost Olivier Giroud, who played a crucial role in the club.
“He was the kind of profile who likes to play with his back to goal, who likes to hold up balls, who creates space for Timo Werner, Kai Havertz, Christian Pulisic, who is ready to fight physically with defenders.
“It’s still not the moment to talk about names, because we have nobody signed yet and we will show all the respect to other clubs.”