After an even first half in which the English side took a 1-0 lead through Michael Owen, United dominated in the second period against a second-string Sounders outfit.
Owen slid a header past Seattle goalkeeper Kasey Keller in the 15th minute after the hosts had controlled the opening moments of the match. It remained 1-0 at the break thanks largely to a brilliant save by Keller on an Owen shot late in the half.
Rooney came on as a second-half substitute and scored in the 51st, 69th and 72nd minutes, while United got further goals from Park Ji-sung, Mame Biram Diouf and Gabriel Obertan.
The match in Seattle was the second stop on United's preseason tour of the United States. They'll go to Chicago for a match with the Fire on Saturday, then face the MLS All-Stars in New York next Wednesday before closing out its tour with a Champions League final reprise against Barcelona.
This was Seattle's only friendly of the season, and for a half the Sounders - on a nine-game unbeaten streak in Major League Soccer - were right in the contest, creating but missing a number of good chances.
But a combination of substitutes and fatigued starters were no match for the Red Devils in the second half.
Rooney roamed the field and enjoyed a degree of marker-free license he is rarely afforded in England. He scored his first goal by blasting a shot into the roof of the net after receiving a pass from Nani. He again was left alone in the 69th minute, collecting a pass from Park, turning and beating Sounders reserve keeper Terry Boss with a shot off the post.
Rooney returned the favor to Park moments later when he let a pass from Obertan slide through to the Korean, and Rooney completed his hat trick by getting the last touch after a scramble in front of the net.
Owen's finish in the 15th minute was as stunning as the cross that came off the foot of defender Patrice Evra. Making an overlapping run on the wing, Evra received a pass from Ashley Young and raced toward the byline. While getting a shove from a Seattle defender, Evra managed to get in a cross to a waiting Owen, who headed past a stunned Keller.
While Keller was beaten on that occasion, the 41-year-old showed his quality when he denied Owen in a one-on-one situation after the United forward beat the offside trap.