Chargers offensive coordinator Ken Whisenhunt worked with Mettenberger as the Titans' head coach the past two years.
The Titans drafted Mettenberger in the sixth round out of LSU by the Titans in 2014. He was in year three of a four-year, $2.345 million rookie contract that he signed with Tennessee.
Last season, Mettenberger played in seven games and threw for 935 yards and completed 60.8 percent of his passes for four touchdowns and seven interceptions while rookie Marcus Mariota was sidelined.
In two years with the Titans, Mettenberger had an 0-10 record as a starter.
The 24-year-old Mettenberger joins a group of quarterbacks vying to back up Philip Rivers with the Chargers. Kellen Clemens served as the backup the past two seasons. Also on the roster are quarterbacks Bryn Renner and Mike Bercovici.