The 33-year-old has put pen to paper on a deal that will keep him at Stade Auguste Bonal until the end of the season.
"I want to come back to lend a hand," Frau, who scored 75 league goals in 207 league appearances for Sochaux between 1997 and 2004, told L'Equipe.
"This is where I was born, where I started, it is a homecoming. Late last week, the coach told me he had been reassured by my physical condition.
"Although, initially, I wanted to wait until January and not rush, I did not hesitate."
Frau has played for Lyon, Lens, Paris Saint-Germain, Lille, Caen and Al-Wakrah since leaving Sochaux in 2004.