Patrice Evra, 36, arrived at the club’s Rush Green training complex on Wednesday morning before passing a medical and signing a short-term contract until the end of June 2018.
Sky Sports News understands Everton were also interested in the former Manchester United, Juventus and Monaco left-back but the player favoured a move to London.
Evra has been assigned the no. 27 shirt and could make his debut against Watford on February 10 at London Stadium.
He told West Ham’s official website: “I’m really happy to be a Hammer, to be back in the Premier League and I love this game!”
“It’s amazing to be back and I am thankful to West Ham for giving me the opportunity to do my job. When I wake up and know I am going to work hard, to have fun with my team-mates and give my best on the pitch, I want to say a massive thanks to West Ham, to the chairman, to the manager to all my teammates, who have given me a nice welcome, my agent and my brother.
“I’m just happy to be here, it’s really simple. You don’t realise how lucky you are and just to wake up this morning and know I am going to train with my new teammates made me smile!”