Branyan signed a one-year contract with the Seattle Mariners on December 3, 2008. The Mariners gave him the starting first base job before spring training. At the end of the season, Branyan was the team leader in home runs. Branyan stated one of the keys to his success with the Mariners was a vision training program with Chicago ophthalmologist Dr. Barry Seiller. The exercises strengthen eye muscles and improve focus at the plate. The program was similar to the one designated hitter (DH) Edgar Martínez used when he played for the Mariners. Branyan hit his 25th home run of the year on August 7, setting a new career high. His previous career-high of 24 had been achieved during the 2002 season. He also set career records in RBIs, doubles, hits, runs and total bases in the 2009 season.