The Hammers spent the latter stages of the transfer window negotiating with the Blues for the centre-half, before sealing the deal before the deadline.
The 26-year-old brings a winning mentality to West Ham, having won two Premier League titles and a Champions League, along with the EFL Cup and UEFA Super Cup titles.
Moyes told the club's website: "Kurt is a player we have tracked for some time, and we have taken great care and diligence to make the transfer happen," the managed revealed. "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."
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