With the score tied 1-1, Andrew Wiedeman sent a cross through the 6-yard box that got past defender Osvaldo Alonso and fell to an open Osario. Goalkeeper Marcus Hahnemann came out to challenge, but Osario's shot bounded off his leg and into the net.
TFC (5-10-8) had erased a 1-0 deficit in the 40th minute on an own-goal by Seattle's Doneil Henry.
The Reds extended their longest winning streak of the season with a third consecutive victory.
Mauro Rosales scored for the Sounders (9-8-4), who fell in the return of U.S. national team captain Clint Dempsey to the MLS for the first time since 2007.