Stade, who all but confirmed their place in the top flight for next season after beating champions Paris Saint-Germain at the Parc des Princes on Wednesday, failed to reach that level in the opening half.
Indeed, Sebastien Roudet's shot on 30 minutes was the only effort on target for Rennes in the first half.
Sochaux looked more likely to score again and Florian Marange may think he ought to have converted at least one of his two chances midway through the second half.
The hosts relied on Paul-Georges Ntep, the hero from Paris in midweek to pull them back on terms 18 minutes from time.
However, the game was won on 77 minutes when Roudet's corner reached the head of Emmanuel Mayuka who netted to the delight of the travelling support.
The result leaves Sochaux needing to win their final game against Evian next Saturday to relegate their opponents.