Ruidiaz scored twice in the second half after Joao Paulo's memorable opener to give last season's runner-up Seattle the perfect start on opening day.
The Sounders – beaten by Columbus Crew in the 2020 MLS Cup final – hit the front four minutes into the second half, Joao Paulo cushioning a cleared cross with one touch before rifling a thunderous volley into the top corner on Saturday (AEST).
A dominant force in the Western Conference since Brian Schmetzer succeeded legendary former boss Sigi Schmid in 2016, the Sounders doubled the lead with 20 minutes remaining when Will Bruin teed up Ruidiaz.
Ruidiaz made it 3-0 three minutes later after Cristian Roldan turned provider before Fredy Montero completed the rout four minutes from time at CenturyLink Field in Seattle.
In the only other match on Saturday, Houston Dynamo edged San Jose Earthquakes 2-1.
Houston built a two-goal buffer by the 55th minute thanks to Memo Rodriguez and Maximiliano Urruti.
A stunning 74th-minute strike from San Jose substitute Paul Marie set up a tense finish, but Dynamo held on for maximum points.