West Ham swoop for Jaaskelainen
The 37-year-old has made a quick return to the Premier League after turning down the chance to remain with the Trotters.
The experienced goalkeeper will move to the Hammers when his current deal expires at the Reebok Stadium.
"I'm delighted to be joining West Ham United," he told the club’s official website. "After spending 15 years in Bolton it's a great new challenge and I'm really looking forward to it.
"Everything has happened really quickly. When the opportunity was offered to come to West Ham I didn't think twice and I just told my agent to sort it out as quickly as possible.
"I appreciate Bolton giving me permission to speak to West Ham before my contract was up and for the efforts the chairman made in trying to get me to stay.
"I would like to thank the fans for all their support over the years and I wish the players and, in particular, Owen Coyle all the best in gaining promotion at the first attempt."
West Ham manager Sam Allardyce had been keen on bringing in a new number one following the release of Rob Green.