Fly-half Ross, 28, spent four seasons with Leeds and amassed almost 450 Premiership points before joining Castres.
He will challenge in-form New Zealander Glen Jackson for Saracens' number 10 shirt next season.
Saracens rugby director Alan Gaffney said: "Gordon is a very good signing for us and will add extensive Premiership and international experience to the squad.
"He is a player we have been interested in for some time and someone who can control the game and get us around the paddock.
"With injuries we've had over the season, there have been times when we've been very light in the fly-half department, so this is a signing that will add a lot of depth to the squad.
"It will also take the pressure off Glen Jackson, but it will also be a challenge for him because Gordon is a very good player in his own right.
"We told Gordon from the outset that we don't want him to come here and be a number two, and it should be a good contest between them."