The 24-year-old has been explicit in front of goals, scoring 43 goals in two seasons for the club to help them rise through to Ligue 1.
“This is a proud day to have signed for West Ham, I really love English football and I’ve dreamed of coming to play for one of the big English clubs. Today, West Ham have given me that opportunity and I didn’t hesitate to grab it,” Sakho told West Ham TV.
“I’m looking forward to playing. It’s more than a month now that I’ve been training for this and right now I think I’d be ready to pull on the Club colours.
“I’ve come here to continue as I was at Metz. I’m going to give it my all, going flat out on the pitch and I think the fans will appreciate that.”