The 28-year old midfielder has completed a move to East London where he has penned a five-year deal with the transfer fee believed to be around £4million.
Nolan reunites with new Hammers boss Sam Allardyce, who he played under at Bolton, and is looking forward to a new challenge with the relegated club.
"It's been a bit of a whirlwind, but obviously once I knew they wanted me, they have shown a certain hunger and desire to make sure I become a West Ham player," he told the club's official website.
"Having the chance to link up with Sam again is a massive thing and obviously coming to such a massive club is great for me.
"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 there.
"I've come to West Ham because of the tradition of the club and everything about it. I've always enjoyed playing here and there was also the chance of linking up with Sam and working with him again."