The longtime Dodgers pinch hitter was dismissed as Dodgers hitting coach on Oct. 12.
Hansen, who was a hitting instructor for the Dodgers in 2011 before getting promoted to hitting coach on July 20 last year when Jeff Pentland was fired, ended his playing career with the Mariners in 2004-2005.
"I had the chance to talk to Dave several times, and have also talked to players and coaches he’s worked with, and we feel he is a great fit for our club," Mariners GM Jack Zduriencik said in a statement. "We look forward to having Dave re-join the Seattle organization."
Hansen was offered another position in the Dodgers' organization, but turned it down.
The Dodgers still haven't named a replacement for Hansen as hitting coach. One possibility was removed on Friday when the Rangers hired Dave Magadan as hitting coach, replacing Scott Coolbaugh.
In other Dodgers coaching news, the Red Sox on Sunday made official their acquisition of manager John Farrell from the Blue Jays. Dodgers third base coach Tim Wallach had interviewed for the job, as did former Dodgers catcher Brad Ausmus. Wallach will return to the Dodgers in 2013.