The reigning European champions forked out €35m (£31m/$40m) plus a season-long loan move for Mateo Kovacic to Stamford Bridge.
Courtois, who has been with the Roman Army since 2011 was named best goalkeeper at the 2018 Fifa World Cup Russia.
Here is how social media have reacted to the 26-year-old’s move, with many tipping Barcelona’s Lionel Messi to score goals aplenty when they face Madrid.
i knew we'd sign Courtois after Messi put the ball between his legs multiple times — Jon Kaan (@DonJonPablo) August 8, 2018
Welcome to Messi’s League ❤️💙Thibaut Courtois
But I’m sorry if you can’t deal with balls like this you better return back to England 🏴.. pic.twitter.com/UPGX47f1TR — Andy Carter (@Thacarter228) August 8, 2018
Welcome, Courtois 🥚 pic.twitter.com/psIavwM0mJ— Ahmed. (@A7mds10) August 8, 2018