The 26-year-old France international ends a seven-and-a-half year stay at Stamford Bridge by moving to the London Stadium.
Zouma joined Chelsea from Saint Etienne in January 2014 and made 151 appearances in all competitions for the Blues, winning two Premier League titles, one League Cup, the UEFA Champions League and UEFA Super Cup.
He also spent time on loan at Stoke City and Everton while on the books of Chelsea.
"I'm very happy and very proud," Zouma told West Ham's official website.
"My conversation with the manager went very fast. I just felt like he really wanted me to come and join the team, especially a good team that's started the season very, very well.
"With the great season that West Ham had last year, that gave the team a lot of confidence to improve this year - and I want to help the team do that."
West Ham manager David Moyes added: "I am delighted to welcome Kurt to West Ham United. He is a player we have tracked for some time, and we have taken great care and diligence to make the transfer happen. He was always our first choice and I am very happy that he is now our player.
"Kurt will join a squad full of players who are hungry and ambitious. He will provide competition to our squad. He is a strong, powerful player with great experience in the Premier League while he is also at a good age for a centre-back.
"Kurt has enjoyed great success at Chelsea and I can see that he is determined to continue being successful here at the London Stadium. Myself, the coaching staff and players are all looking forward to working with him and we wish him all the very best in his career with us."
Zouma has won eight caps for the France national team.
