The Hammers have secured the 24-year old Senegalese striker on a four-year deal for an undisclosed fee, believed to be around £4.5million.
Sakho scored 20 goals for Metz in France’s Ligue 2 last season, and 19 the season before, and is planning to bring that form to Upton Park for the 2014/15 Premier League season and beyond.
"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's official website.
"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."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball