Fenby, who can play on the wing or at full-back, has been a free agent since leaving London Irish at the end of last season.
The 30-year-old initially announced his retirement from the sport, but will now return on a short-term basis to help Saracens after they were hit by a spate of injuries and suspensions.
"He is an experienced player. We feel that he can make a valuable contribution," director of rugby Mark McCall told the club's official website.
"With a number of injuries and suspensions to our backline, we are very happy to bring a player of Andrew's talent into the group. He's 30 and has played over 100 games in the Pro12 and Premiership."
Sarries currently sit second in the Premiership table with four wins from their opening five games.