The victory lifted Seattle (10-7-7, 37 points) into a tie for the top spot in the Western Conference with Sporting KC (9-5-10, 37 points).
After testing the Sporting defense for much of the first half, the Sounders finally found the back of the net in the 36th minute. Jordan Morris crossed from the right side to Dempsey, whose header put the Sounders in front. Sporting goalkeeper Tim Melia leaned to his right before diving back to his left, to no avail.
Seattle finished the half with a 7-2 advantage in shots, and a 50/50 split in possession. Sporting controlled 58 percent of the possession in the second half, but had few chances to test Seattle goalkeeper Stefan Frei. Sporting had just three shots on goal the entire match. The Sounders now own a franchise-record 395-minute shutout streak.
Sporting's road struggles continued. Through 12 road contests this season, SKC has scored a total of six goals.
This was a matchup of two hot clubs. Sporting came into the match riding a nine-match unbeaten streak in MLS play (13 straight matches in all competitions), while Seattle extended its unbeaten streak in MLS play to eight straight.
The victory ended Seattle's six-match winless streak against Sporting KC.
Sporting had almost an entirely new starting lineup from the one that faced the San Jose Earthquakes in the Lamar Hunt U.S. Open Cup on Wednesday night, a match that went 120 minutes and was decided in penalty kicks. Only Melia and Matt Besler started both matches.