Dempsey got his first taste with his new club from the stands as he cheered on first half goals from Obafemi Martins and Eddie Johnson. Seattle would get a third goal in the final minutes of the game with a penalty kick from Brad Evans.
The loss extends FC Dallas' winless run to nine games as well as extends the club's scoreless drought to five games.
The night was all Seattle's from the opening whistle. Just a few minutes into the match, the Mauro Rosales sent a ball into the penalty box that found a wide open Martins for the game's first goal. His header kept the big crowd at CenturyLink Field rocking, as if they needed any more after the unveiling of Dempsey.
The Sounders would find a second goal not long after their first. Thanks to a high defensive line and lack of pressure in the middle of the field, in the 17th minute Brad Evens sent a long ball over the top to Johnson, the former Dallas striker did the rest as he easily beat Raul Fernandez on the one-on-one breakaway.
Dallas saw few chances throughout this one as the midfield struggled to keep the ball and keep pressure on the Sounders. The few chances Dallas was able to get was thanks to their pace on the wings with Fabian Castillo and Jackson, but each time both got into the box early on, they weren't able to test the Seattle keeper.
The second half came with a couple changed by Schellas Hyndman as he brought in Mauro Diaz and Kellyn Acosta, both making their MLS debuts. Unfortunately for Dallas fans neither of them found the back of the net on the night.
Diaz did show a few signs of why the club signed him last month. His best chance came just after the hour mark of the match when he took a good pass from David Ferreira at the top of the penalty box, made a cut to his left and took a shot that went just wide.
Ferreira would lose his cool in the final moments of the match, bringing down a Seattle player in the box, costing his club a penalty. Evans would hammer it home to ice the game for the Sounders.
Dallas will return to league action next Saturday as they host the LA Galaxy.
(More from this one coming up)