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.
https://twitter.com/TetoBarca_/status/1027278605905215488
https://twitter.com/FranciaMessi/status/1027277854336331778
https://twitter.com/MessiWorId/status/1027279951043719169
i knew we'd sign Courtois after Messi put the ball between his legs multiple times
— Jon K (@DonJonPablo) August 8, 2018
https://twitter.com/HazardFIicks/status/1027278053196656640
https://twitter.com/MessiZoneV3/status/1027279362746470400
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— A N D Y C A R T E R (@1Andrew_Carter) August 8, 2018
Welcome, Courtois 🥚 pic.twitter.com/psIavwM0mJ
— Ahmed. (@A7mds10) August 8, 2018
When Lionel Messi heard Thibaut Courtois has joined his foundation. pic.twitter.com/uIFeOau3oR
— 👑King Boluwatife 👑 OF ABJ (@kingboluwatife) August 8, 2018
News: Courtois joins Real Madrid!
Messi's wife: pic.twitter.com/HE9woG7LNu— Gaa (@gaa_jp) August 8, 2018