Rooney stars as United thrash Sounders
Michael Owen set the tone in the 15th minute, beating former Premier League foe Kasey Keller with a header to open the scoring. It remained 1-0 at half-time largely thanks to a superb save by Sounders keeper on Owen late in the half.
After the interval, however, the English Premier League giants took charge with the addition of second-half substitutes including Rooney, Park Ji-Sung and Mame Biram Diouf.
The latter scored just four minutes after the break, while Rooney scored in the 51st, 69th and 72nd minutes. Park squeezed in a goal in the 71st, and Gabriel Obertan completed the scoring in the 88th.
The match was the second stop on United's pre-season tour of North America. They now head for a match against Seattle's Major League Soccer rivals Chicago Fire on Saturday, and will take on the MLS All-Stars in New York next Wednesday. United will wrap up their tour with a UEFA Champions League final rematch against Barcelona on 30 July in Washington.
Seattle, who are on a nine-game unbeaten streak in the MLS, kept it close for a time, missing several scoring opportunities.
Rooney, roaming the pitch, found himself unmarked early in the second half when he took a pass from Nani and lofted a shot over back-up keeper Terry Boss.
He was alone again when he received a pass from Park, turned and beat Boss with a left-foot shot. Rooney capped his night by slotting home the ball in a scramble in front of the goal.