Sigi Schmid's men got a late double from Marco Pappa to ensure they took home their second trophy of the season
After a 2-2 draw between the two sides last week in Los Angeles, Seattle only needed a draw to clinch the Shield, while the Galaxy needed a win.
The first half was a scrappy, disjointed affair, with neither side really testing the opposing goalkeeper much. Referee Mark Geiger was the busiest man on the pitch, awarding 21 fouls and giving out three yellow cards.
The second half continued in much the same fashion, with the highlight a mini brawl that the MLS disciplinary committee may have to examine going into the playoffs, as multiple players appeared to throw punches. Geiger only produced yellow cards for Gonzalo Pineda and Marcelo Sarvas, however.
Seattle sewed up the Shield with just five minutes to play. Off a quick free kick, Obafemi Martins found Marco Pappa inside the box, and the Guatemalan finished low past Jaime Penedo.
There was still time for Seattle to add further insult, as Pappa swiped the ball from Penedo – who made a shocking decision to try to dribble his way out of trouble 25 yards out from his goal – and finished into an open net deep into stoppage time.
With the win, Seattle are now two-thirds of the way to becoming the first MLS team to win the treble. The Sounders won the U.S Open Cup last month, and now only need MLS Cup glory to complete the hat-trick.