The Padres have dropped six in a row and 10 out of 11. They fell to 3-13 in inter-league play by losing to the team with the worst record in the majors.
Washburn (3-7) finally got a victory after having suffered through three straight no-decisions because of little run support. He gave up only a two-run homer to Chase Headley in the seventh that cut the Mariners' lead to 4-2.
Brandon Morrow retired the final four Padres for his third save.
Beltre's two-run single finished off a three-run fourth and his infield single in the ninth drove in Willie Bloomquist.
The loss went to Randy Wolf (5-7), who allowed three runs on eight hits in 3 1/3 innings.