Introduced as a second-half substitute, Rooney quickly cut loose, drilling home his first just after the re-start and completing his treble 18 minutes from time.
It was part of a seven-goal romp which takes the Red Devils' total in two games so far to 11, with Michael Owen, Mame Biram Diouf, Park Ji-sung and Gabriel Obertan netting the rest.
The only downside was a knock picked up by Rio Ferdinand near the end that left the defender limping at the final whistle.
United fly on to Chicago on Friday before further engagements in New Jersey and Washington and with just over three weeks to go before they begin their Premier League title defence, Sir Alex Ferguson's men have already served notice they will not easily be dislodged from their status as England's number one.
United had been promised a different type of atmosphere in Seattle and they were not disappointed as in excess of 67,000 supporters showed their appreciation for their visitors, whilst leaving no doubts over who they wanted to win.
With David de Gea not pushed into the fray just yet, Anders Lindegaard took his opportunity to impress, denying Alvaro Fernandez, then producing a fine double save in repelling Fredy Montero and James Riley.
Indeed, it was somewhat against the run of play that United went in front through a typical Owen poacher's effort.
Getting himself into space as Patrice Evra ran onto Ashley Young's lay-off, Owen met the Frenchman's low cross perfectly and quickly stabbed it past Kasey Keller.
Owen has looked impressive in pre-season so far and, starting alongside Federico Macheda, had another effort cleared off the line by Osvaldo Alonso.
That was another piece of opportunism but there were no excuses for the chance he spurned three minutes before the break, when he was brilliantly sent through by Ryan Giggs.
Owen steadied himself but Keller read his mind, the former Tottenham keeper making a decent stop.
Sir Alex Ferguson made four changes at the interval and within three minutes, Diouf had got his name on the scoresheet.
Supposedly attracting interest from Blackpool, the Senegal-born player ran onto Nani's through ball before turning sharply and drilling home from an acute angle.
Fellow substitute Wayne Rooney added another shortly afterwards, with Nani again the provider and the England striker added another midway through the second half as he rolled a precise finish into the corner.
By now, United were really into their stride and Rooney set up Park for their fifth before completing his hat-trick from Obertan's cut-back.
The Frenchman has not made that much of an impression in his time at Old Trafford to date but he rounded off the romp two minutes from time.