Hammers manager Sam Allardyce confirmed on Friday afternoon that the Moroccan has completed his move across London - but not in time to figure in Saturday's FA Cup third round tie against Manchester United at Upton Park.
Allardyce told Sky Sports News: "We've signed him for the rest of the season. I think that'll be it in terms of strikers as we need to get Andy Carroll back (to fitness)."
Chamakh, 28, struggled to claim a regular place in the Gunners' first team after initially impressing following a free transfer from Bordeaux in 2010.
Chamakh hopes a spell at Upton Park can kickstart his career following a frustrating spell on the fringes of the Arsenal team.
Speaking to the Hammers' official website, www.whufc.com, the Morocco forward said: "I want to play and give my best for this team. I can bring my heading, some goals and we will see what else.
"I didn't have opportunities to play recently, but I did well before and I know I am a very good striker.
"We played only one striker at Arsenal, so I didn't play a lot, so I hope to do so more with West Ham."
"I think this will be a very important move for me and I don't want to waste any more time. I want to contribute immediately."