Sochaux took the lead in the 25th minute when an unmarked Stopilla Sunzu nodded the ball home at the far post from a deep cross.
Mathieu Valbuena sent a fizzing shot inches past the post 10 minutes after the goal, and the diminutive winger shaved the post shortly after with another effort.
Sochaux almost doubled their lead shortly after the break, but Lucas Mendes was alert enough to clear an effort off the line.
Marseille pushed the hosts further and further back into their half as the game went on, but it looked as though their chances of rescuing a point had disappeared when Alaixys Romao hit the bar.
However, Nkoulou swept the ball home in 90th minute to ensure the visitors took a point.