The Sweden international, 30, had been the Gunners' longest serving player after costing Arsene Wenger's side $7.5million in 1998 from Halmstads.

Ljungberg joins long time team-mate and star striker Thierry Henry, who signed for Barcelona in June, in leaving the club this summer.

The midfielder played 325 times for Arsenal, scoring 72 goals, but lost his grip on a starting spot last season.

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.

Ljungberg said he left Arsenal because he felt like "it was the end" for the club as he knew it following Henry's move.

"Two years ago when I signed my last contract we talked a lot about the future and about bringing great players to the club. Despite building the new stadium we wanted to stay top in England and do well in Europe.

"For me that didn't really happen.

"I stayed because I wanted to stay loyal to Arsenal. But when Henry left this summer it felt like the end for me unfortunately.

"I felt like all the unbeaten players (from Arsenal's 'invincibles' of 2003/04) had left. I wanted to be loyal but I felt like it was the right time.

But Ljungberg also insisted there were positive reasons behind his decision and insisted he shared owner Eggert Magnusson's vision.

"I'm coming here because I want to build a great team.

"And why I want to join is because of Magnusson. He wants to bring them forward to be top of the country.

"He's willing to go to the transfer market to buy top players and mix them with youth players. West Ham have a great academy.

"He wants to help take the club up and take them to the top in England. It's a big challenge for me."