The brilliant Belgian striker re-joins the west London club from Inter Milan, where he reclaimed his status as one of the best footballers on the planet.
In 95 games across all competitions for the Italian outfit, Lukaku netted 64 goals and contributed 16 assists, a record any forward would be incredibly proud of.
Chelsea didn't score enough goals last season, especially in the Premier League, but you get the feeling that won't be an issue in 2021/22.
The acquisition of Lukaku really could be a game-changer for the Blues and it'll be a huge shock if they don't challenge Manchester City for the title.
Chelsea sign Romelu Lukaku | Football Terrace
With the Belgian striker now back at Stamford Bridge, Chelsea boast arguably the best squad depth of any team in English football right now.
We've created a graphic to highlight that fact, which you can view below.
Chelsea's insane squad depth for 2021/22
Imagine having the likes of Christian Pulisic, Hakim Ziyech, Timo Werner and Mateo Kovacic in reserve...
Is Chelsea's squad depth better than City's right now? There's certainly a case to be made that it is.
The Blues are going to be a serious threat on all fronts next season and you wouldn't bet against them retaining the Champions League title they won in 2020/21.
Chris Sutton believes that the signing of Lukaku makes Chelsea the favourites for the Premier League crown - unless City manage to bring in Harry Kane from Tottenham.
"It would make Chelsea favourites for me, if Manchester City didn’t sign Harry Kane," Sutton said on the Football Daily Podcast a few days ago.
"I think Chelsea are in a brilliant, brilliant position if Lukaku signs and he could well be the difference.
"He's been so, so prolific and I think that Chelsea need that reliability from a centre-forward."