Fabian Delph has joined Manchester City for £8 million from Aston Villa.
The England international, who had vowed to stay at Villa Park six days ago, has agreed a five-year deal at the Etihad Stadium.
He completed a surprise switch after passing a medical on Friday, leaving the FA Cup finalists for the Premier League runners-up.
Delph becomes City's third summer signing after 18-year-old Turkish striker Enes Unal, who cost £2m, and Liverpool winger Raheem Sterling, who arrived for a club record £49m, and his arrival takes their summer spending to £59m.
- Manchester City FC (@MCFC) July 17, 2015
The 25-year-old provides City with midfield reinforcements after the summer departures of former Villa player James Milner and Frank Lampard.
Manager Manuel Pellegrini hailed his new signing as one of the finest midfielders in the league.
Pellegrini told City's official website: "Fabian Delph is an excellent player, and I am looking forward to working with him.
"He has made a huge impact in recent seasons at Aston Villa, and is now one of the brightest young midfielders in the Premier League.
"Fabian has tremendous skill and energy, and the fact that he has become a regular member of the England squad is further evidence of his progress over the past few seasons. At just 25 years old, he still has his best years ahead of him."
Delph, who was appointed Villa captain by Tim Sherwood, had scored eight goals in 128 games for them after his £6m arrival from Leeds in 2009.
But he leaves at a time when Villa are also likely to lose top scorer Christian Benteke, who is set to join Liverpool for £32.5m.
Delph had been expected to join City earlier but then promised to stay with Villa last week, when he told their club website: "I'm not leaving. I'm staying at the football club and I can't wait for the start of the season and captaining this great football club."
On Friday, Delph called leaving Villa "the hardest decision I've had to make."
Delph added in a statement: "My decision to stay initially was purely based on the emotion and feelings that I hold for the club and the people there that I would be walking away from -- rather than looking at the football opportunity that had been placed in front of me."
He will face a furious backlash from the Villa side with former striker Stan Collymore branding the player "an absolute disgrace" for performing such a quick U-turn on his public loyalty pledge.
Speaking before the deal was officially confirmed, Collymore told talkSport's Drive programme: "He said twice now that he wants to stay at the club -- nobody put a gun to his head.
"He said twice that he believes Villa is a great club that he wants to stay at... if he signs for Manchester City or any other club in the next few days then he's an absolute disgrace."
Lampard, who had himself angered some at his former club Chelsea by joining City on loan shortly after signing for sister club New York City FC wished him luck:
"Congratulations and good luck to Fabian Delph, the new number 18 @MCFC. Enjoy wearing the shirt for City and England!" - Frank Lampard
- New York City FC (@NYCFC) July 17, 2015
Information from Press Association was used in this report.