The 30-year-old Swede, the Gunners' longest-serving player since joining in 1998, follows star French striker Thierry Henry, who moved to Barcelona, in leaving the Premiership club this summer.

The midfielder played 325 times for Arsenal, scoring 72 goals, but lost his grip on a starting spot last season. No transfer fee or length of contract was immediately given.

He now joins Julien Faubert, Scott Parker, Craig Bellamy and Richard Wright in moving to Upton Park since the Hammers' dramatic last-day survival last season.