The 29-year-old, who has received one France cap, becomes the Hammers' ninth signing of the summer after joining for an undisclosed fee.
Amalfitano spent last season on loan at Premier League rivals West Brom, scoring four goals - including a memorable strike in the Baggies' 2-1 win against Manchester United at Old Trafford.
"It's very good news and I'm very happy to come to this big club," he said.
"I've waited a long time, but I am very happy to have signed here. The club wanted me a lot and for that I waited for West Ham and now I'm here.
"The manager explained to me how the team plays and how he needs me. I explained to him where I prefer as my position in his team. This is a new team with very good players with great experience of this league. I think everything can go well and I hope we have a good season.
"I prefer to play behind the striker, but last year with West Brom I played on the right. I like to cross the ball too, but I can also give through passes from behind the striker. The coach will see what happens in every game and if he finds another position for me in the team, there is no problem for me.
West Ham had been expected to bring in defensive recruits on transfer deadline day but Amalfitano instead bolsters manager Sam Allardyce's ranks.
He joins recent loan acquisition Alex Song, with Carl Jenkinson also recruited on a temporary deal from Arsenal. Permanent moves for Enner Valencia, Diafra Sakho, Cheikhou Kouyate, Diego Poyet, Aaron Cresswell and Mauro Zarate were secured earlier in the transfer window.
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