The former Magpies captain has signed a five-year deal after being sold for an undisclosed fee thought to be in the region of £4m. He had decided to leave St James’s Park after failing to agree on a contract extension with the club.
The move reunites the 28-year-old with his former Bolton manager Sam Allardyce.
“Having the chance to link up with Sam again is a massive thing,” he told the Hammers' official site. “West Ham have shown a certain hunger and desire to make sure that I became their player.
“I’m very sad to leave Newcastle but I’m coming to another massive club in my eyes and hopefully I can be as successful as I was [at Newcastle].”
Newcastle boss Alan Pardew told the Shields Gazzette: “I would like to thank Kevin for his sterling efforts as a player and captain of this club, especially since I took over last season.
"He displayed admirable leadership qualities, and I wish him every success at West Ham. It’s a great club and I’m sure he will thrive there.”
Nolan, who scored 12 league goals last season, joined Newcastle from Bolton for £4m in 2009.