Second-half goals from Obafemi Martins and Chad Marshall allowed the Sounders to come from behind to edge the Union at home after Brad Evans' 13th-minute own-goal.
Seattle have now won six matches from nine this season, including four in a row, to be within one win of equalling a franchise record.
The Sounders have won five times in a row on three occasions and if they manage to overcome FC Dallas later this week, will go within one win of setting a new club record.
Last season, the Sounders streaked five consecutive wins between August 25 and September 13 to equal the record set on two other occasions in 2010 and 2012.
Sigi Schmid's side currently sit on top of the Western Conference table with 19 points from nine fixture, which has them two points clear in their region, and on top of the overall MLS standings above Real Salt Lake.
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