The England fullback, 28, joins West Ham after coming off contract at Southampton this summer.
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"He is a player I've always thought highly of and been impressed by whenever he has faced one of my teams," said West Ham manager Graham Potter.
"He has a really good pedigree, a top professional who has established himself as a respected Premier League player and still has his best years in front of him."
Walker-Peters also said: "I spoke to Graham Potter and he was a big, big part of me making the decision.
"I've always liked his style as a coach, and I think it will suit me well.
"Every team wants to win trophies and West Ham have that ambition. From speaking to Graham, he wants to create something special, and hopefully I can be a part of that."