Seattle had won three straight titles, from 2009-11.
In the 75th minute, Hristov tapped in a rebound after Marcus Hahnemann made a diving save on Luke Mulholland's shot deep inside the box. It was the third goal of the tournament for Hristov in just two games.
The Sounders nearly equalized late when they were awarded a penalty kick in the 84th minute after Jordan Gafa's hand ball in the box. Obafemi Martins' ensuing attempt was saved by Tampa Bay keeper Diego Restrepo.
Gafa was issued a red card on the play and the Rowdies played the game's final minutes with 10 men.