Jackson confirmed the signing via his Twitter account Thursday night.
The move had been widely expected since Wednesday, when ESPN.com's Adam Schefter reported that Jackson was likely on his way back to Seattle after being released by the Buffalo Bills on Monday.
"I think it's a great boost for us in a competitive sense," Seahawks coach Pete Carroll said after Thursday's workout, according to ESPN 710. "We thought of Tarvaris as a tremendously tough football player and competitive kid that battled for us."
Jackson started 14 games for Seattle in 2011, but he lost a three-way quarterback competition with rookie Russell Wilson and free-agent signee Matt Flynn before last season and was traded to Buffalo. Seattle was in the market for a backup quarterback after trading Flynn to the Oakland Raiders.
Jackson has started 34 games in his career. In his one season with Seattle, he threw for career bests of 3,091 yards and 14 touchdowns.
All the hawk fans that welcomed me back I appreciate it... Feels great to be back in the great northwest #letsgoseahawks — Tarvaris Jackson (@7tjackson) June 14, 2013