Terms of the deal were not disclosed.
Unrein played at the University of Wyoming before signing with the Houston Texans as a free agent in 2010. He spent five seasons with the Broncos (2010-14), including most of 2010 on the Broncos' practice squad, after he was released by the Texans.
Over the last four seasons, Unrein played in 54 games with three starts and recorded 48 tackles and a fumble recovery. He also appeared in six postseason games.
In 2012, Unrein saw action as a fullback in short-yardage situations and caught a 1-yard touchdown pass in a game against Tampa Bay to become the first Broncos defensive lineman in team history to catch a touchdown pass.
The Chargers now have 11 defensive linemen on the team's roster -- Damion Square, Tenny Palepoi, Corey Liuget, Ricky Tjong-A-Tjoe, Kendall Reyes, Ryan Carrethers, Sean Lissemore, Ricardo Mathews, Damik Scafe, Chas Alecxih and Unrein.