West Ham have been pursuing the winger all summer, and finally got their man after Jarvis passed a medical in London before signing a contract until 2017 with the option of a further year.
"I'm delighted to have signed," he said. "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.
"I've been told about how good the fans are hopefully they'll get behind me and I can show them what I can do.
"It'll be straight in and hit the ground running. There's no time to think about it, you've just got to get on with it, which is good. I hope to train with the team today and then travel up to Swansea."
West Ham manager Sam Allardyce said he believed the deal would add an extra dimension to his newly-promoted outfit.
"From my point of view it is a big piece of business. I think a quality young man who has got a lot of qualities that will add to the team we have already got," he said.
"When you are trying to create in the Premier League and score goals, he is one of those players who can certainly give defences a really hard time and that is what we are hoping he can do for us."
