Gallagher joined Chelsea at the age of eight and went on to make 95 appearances for the club.
The 24-year-old, who has 18 caps for England, was put up for sale by Chelsea despite captaining the team regularly last season.
A move to Atletico threatened to fall through earlier this month but the £33m transfer was completed on Wednesday, with Gallagher signing a five-year deal at the La Liga club.
‘To everyone at Chelsea, thank you for making my dreams come true,’ Gallagher said on Instagram.
‘It’s been an absolute honor every time I put on the shirt, and it was a dream come true to captain the team on many occasions. I loved every moment.
‘These memories will last forever. I appreciate all the love and support from the fans. Hearing the chant of my name at the Bridge is a special feeling, and the banner you displayed meant the world to me.
‘Thank you for everything. I wish the club all the best for the future, and I hope to see you all soon at Stamford Bridge! Conor.’
Gallagher played in all but one of Chelsea’s Premier League games last season, becoming a key player under former manager Mauricio Pochettino.
But Chelsea decided to sell the England star after continuing their incredible spending spree this summer, signing the likes of Pedro Neto, Tosin Adarabioyo, Omari Kellyman and Kiernan Dewsbury-Hall.
The Blues are expected to offload several more first-team players before the summer transfer window shuts at the end of the month, with Raheem Sterling and Ben Chilwell likely to leave after being left out of their Premier League opener against Manchester City.
Gallagher’s form last season saw him included in Gareth Southgate’s England squad for Euro 2024, though he struggled for game-time during the Three Lions’ run to the final.
More to follow…
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