With the game tied at 1-1 at the end of regulation and extra-time, Sporting kept their nerve best to take the shootout in front of a sell-out crowd at LIVESTRONG Sporting Park in Kansas City.
Kei Kamara, Matt Besler and Paulo Nagamura were all successful with their spot-kicks in the shootout and when Seattle striker Eddie Johnson put his attempt over the bar, the title was decided.
Earlier, Kamara scored the opening goal of the game with just six minutes left in regulation when he converted from the penalty spot.
Zach Scott was adjudged to have handballed in the area and Kamara stepped up to convert the spot-kick.
But two minutes later Seattle were on level terms when Scott headed home a Mauro Rosales free-kick to make it 1-1.
There were no goals in either of the two 15-minute extra-time periods, leaving Sporting to clinch the trophy from the penalty spot.
The victory earns Sporting a berth in the 2013/2014 CONCACAF Champions League, where they will take on the best clubs from North America, Central America and the Caribbean.