Jozy Altidore scored a consolation deep into stoppage time but it was too little too late for the Canadians, who would've felt hard done by after a first-half in which they had chances to score.
As did the Sounders, who pushed on the second half and converted those chances in tangible rewards.
It took 13 minutes after half-time for Leerdam to find the opening goal, although his shot was clearly heading off target before taking a wicked deflection off a Toronto defender and past a helpless Quentin Westberg.
Although the 69,724 crowd - an all-time record at Century Link for any sport - were anything but quiet up until that point, the goal only spurred them on and it didn't take long before mayhem ensued again when Rodriguez doubled the advantage.
After coming on for Australia international Brad Smith, the pacy winger played a sublime one-two with Ruidiaz and subsequently sent an inch-perfect shot into Westberg's bottom far corner.
And it wasn't to be the last time the Emerald City supporters had something to celebrate when Ruidiaz continued his fine form this postseason to make it 3-0.
The Peru international shrugged off a defender to latch onto Gustav Svensson's wishful through ball, showing supreme skill to loft the ball over Westberg for his fourth goal of the play-offs.
Altidore, who overcame injury to make his first appearance in the play-offs, scored with a pinpoint header to peg one back for Toronto.
It was the third time these two sides have met in the MLS Cup in the last four years, and it is Seattle who can now take bragging rights after winning their second title since they entered the league only nine years ago.