Seattle announced the signing on Wednesday afternoon, just days before the start of training camp. Unger, who was a second-round pick out of Oregon in 2009, was entering the final year of his initial rookie contract and was due to become a free agent after the 2012 season.
Terms of Unger's deal were not released.
Unger has started all 32 games he's played in his NFL career. Unger started his entire rookie season in 2009, but missed most of the 2010 season with a toe injury. Last year, he started 15 games at center.