The 24-year-old joins the Hammers for a fee in the region of £4.5m, and cannot wait to get started at Upton Park.
"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," the Daily Star quotes the player as saying.
"Today, West Ham have given me that opportunity and I didn't hesitate to grab it. 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."
Sakho netted 43 times in the last two seasons, helping Metz win promotion back to Ligue 1 after a spell in the French second tier.