By SABC Sport
26th January 2024
The England midfielder will spend the rest of the season at the London Stadium.
The 28-year-old has been frustrated by a lack of game time at City since joining the club from Leeds in the summer of 2022.
Phillips told the Hammers' website: "I'm really pleased to be here. There's always been a lot of speculation about me moving to West Ham, so I'm really excited to get going now.
"There's so much to look forward to in the second half of the season for the club and I can't wait to be part of it.
"There's an unbelievable group here and if I can come in and add the quality that I know I'm capable of, I'm sure we can give the West Ham fans plenty to be excited about between now and the end of the season."
It had been reported the deal could include an option for West Ham to buy the player in the summer but it is understood that was not included in the final agreement.
Phillips, who has made just two starts for City this season, will also hope the move can help him maintain his place in the England squad ahead of Euro 2024 this summer.
Phillips has 31 international caps and played in the last two major tournaments.
His performances for his country and home-town Leeds led to his £42million switch to City but, after an injury-hit first campaign at the Etihad Stadium, he struggled to establish himself under Pep Guardiola.
West Ham manager David Moyes said: "We are really pleased to bring a player of Kalvin's quality to West Ham United.
"We've been big admirers of him for a long time and we believe that he will add strength and competition to our squad.
"Kalvin is an England international midfielder, with proven Premier League experience. We're excited to welcome him into the group and look forward to working with him."
West Ham are not in action this weekend, having been eliminated from the FA Cup, but Phillips could make his debut in next Thursday's home Premier League clash against Bournemouth.