The move was one of several announced by the Chargers on Tuesday. San Diego received a roster exemption for first-round pick Joey Bosa, who just signed on Monday, so the Chargers' roster is at 76 players.
Mettenberger had been outplayed by undrafted rookie Mike Bercovici during training camp, and he did not play in the past two preseason games for the Chargers.
The Chargers claimed Mettenberger off waivers after the Tennessee Titans released him in May. A sixth-round selection in the 2014 draft, Mettenberger finished 0-10 as a starter in Tennessee.
Mettenberger had a relationship with Chargers offensive coordinator Ken Whisenhunt from his time as the Titans head coach. Whisenhunt had a hand in selecting the LSU product.
Mettenberger, 25, was brought in to compete for the No. 2 quarterback job with Kellen Clemens, but that competition never really materialized.
The Chargers were enamored with Mettenberger's physical tools -- he's 6-5 and 225 pounds -- but it did not translate to production on the field.
In his only preseason action, Mettenberger completed 2 of 8 passes for 22 yards, with no touchdowns and one interception against Tennessee. He also was sacked twice.
The Chargers now have three quarterbacks on the roster: starter Philip Rivers, Clemens and Bercovici.