Fredy Montrero had an assist for Seattle FC (1-2-3), which has one win and three ties in its last four games.
Carlos Ruiz gave Philadelphia the lead in the 32nd minute. After Kyle Nakazawa was fouled, Ruiz lined up for the free kick and connected from about 25 yards out, sending the ball past diving Seattle goalkeeper Kasey Keller.
It was Ruiz's second goal in league play this season for the Union (3-1-1), who lead the Eastern Conference with 10 points.
Philadelphia goalkeeper Faryd Mondragon, who entered with a league-low 0.25 goals against average, had three shutouts in the first four matches. He has allowed only two goals all season.