The 26-year-old was expected to complete his move to Upton Park after undergoing a medical with the former Championship side yesterday.
It is understood that Jarvis, who will wear the number seven shirt, was snatched by the Hammers for a club-record fee of around £10m.
"I'm delighted to have signed," Jarvis told the club's official website. "It's a fantastic club and I'm pleased to be here. It's a great club and it's got great history and tradition. I know about Bobby Moore and the history and tradition that go with that.
"I'll be playing with some fantastic players and it'll be great to get into training and I'm looking forward to playing with them. Hopefully I will add a bit of pace, crossing and deliveries that will provide some assists."
West Ham boss Sam Allardyce added: "From my point of view it is a big piece of business for a terrific young man who will add a lot of quality to our team. Certainly from an attacking sense he is one of the best final-third crossers in the Premier League."
Jarvis spent five years at Wolves, who were relegated to the Championship last season.