Atlético Madrid have completed the signing of midfielder Conor Gallagher from Chelsea in a deal worth around €40 million ($44m), the LaLiga side announced on Wednesday.
The transfer had looked in jeopardy after Atlético forward Samu Omorodion's move in the opposite direction fell through.
Gallagher spent five days in Madrid earlier this month -- with Atlético confirming that he was at their Metropolitano Stadium, as his signing was being finalised -- but was forced to return to the UK after Omorodion's transfer collapsed.
After a week of talks, Chelsea agreed a deal to sign João Félix from Atlético instead, allowing Gallagher's transfer to be completed. He has signed a contract until 2029.
Gallagher said a personal goodbye to Chelsea in a post on social media.
He said: "To everyone at Chelsea, thank you for making my dreams come true. It's been an absolute honour 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!"
Sources told ESPN that the England midfielder rejected a new contract offer to stay at Stamford Bridge. Chelsea's last proposal to Gallagher was a two-year deal with an option for a third season and included a significant pay rise.
The 24-year-old had previously turned down a move to Aston Villa while Tottenham had also been monitoring his situation for several months.
Gallagher came through Chelsea's academy and established himself in the first team after a successful loan to Crystal Palace in the 2021-22 season.
He played 136 games in the Premier League, scoring 18 goals and providing 13 assists.
The LaLiga club presented its summer arrivals -- including Gallagher, and eight new signings for the women's team -- to thousands of fans at an event at the Metropolitano on Wednesday.
"I don't speak a lot of Spanish. I'm very happy," Gallagher told the crowd. "Come on Atleti!"
Atlético have had a busy summer in the transfer market, also signing defender Robin Le Normand and forwards Alexander Sorloth and Julián Álvarez.
Information from ESPN's James Olley, Rodrigo Faez and Alex Kirkland contributed to this report.