The 28-year-old will stay at the Roazhon Park till June 2020 after agreeing a two-and-a-half year deal.
Sakho joined the Hammers from Metz in the summer of 2014, and went on to score 10 Premier League goals in the 2014-15 campaign. After struggling with injuries and form, the right-footed attacker garnered 18 goals in 62 Premier League games.
The Senegalese forward expressed joy in joining the Red and Blacks who worked hard in securing in his service.
"I am very happy to join Stade Rennais FC and Ligue 1. I never wanted to join another club than Rennes,” Sakho told the club website.
“The President (Olivier Létang) has worked hard for my coming. I felt it and his speech convinced me to join the club. It is a historic club, structured with great ambitions. I also spoke with the coach (Sabri Lamouchi) and talked about the sports project. It's a great joy and a great pride to join Stade Rennais FC.“I hope to bring my qualities to the group and participate in the success of the club in the second half of the season and for the next."