A fortuitous deflected goal from Kelvin Leerdam, substitute Victor Rodriguez's stellar strike, and a late Raul Ruidiaz tally guided the Sounders to a 3-1 victory in the rubber match on Sunday at CenturyLink Field in Seattle.
Jozy Altidore returned from a quad injury to head home a consolation goal for the visitors in injury time.
With the largely muddied match approaching the hour mark, Toronto continued to dominate possession and looked most likely of the two sides to score.
Leerdam's opener appeared to reinvigorate the Sounders. Suddenly blessed by an attacking impetus, Brian Schmetzer brought on Rodriguez in the 61st minute, and the Spaniard's introduction paid immediate dividends.
Rodriguez doubled the advantage 15 minutes after coming on when Nicolas Lodeiro's slick one-touch pass afforded his teammate the space to fire a thumping right-footed effort from just outside the area beyond a lunging Westberg.
Ruidiaz added insult to injury for 2017 MLS champs Toronto when the pint-sized Peruvian striker beat Chris Mavinga to a deep ball over the top before bagging his fourth goal in four matches.
The Sounders are now one of six MLS sides to win the league's showpiece on two or more occasions.