Terms of the deal were not released.
The Lions and Culpepper had reached an agreement during the weekend but the contract was not signed until after the 31-year-old quarterback underwent a physical exam.
Detroit is 0-8 this season after losing 27-23 to the Chicago Bears in a close game Sunday.
Culpepper has played nine years in the NFL, mostly with the Minnesota Vikings. He most recently was with the Oakland Raiders, appearing in seven games in 2007.
The Lions have recently had Dan Orlovsky at quarterback. Orlovsky has thrown five touchdown passes and has been picked off four times since he replaced Jon Kitna, who was injured and lost for the season.
Culpepper has thrown for 22,422 yards and 142 touchdowns in the NFL. He has been intercepted 94 times.