He opened the scoring for the Red Devils in the 56th minute and went on to make two of his side's other goals. For good measure also had a first-half effort disallowed.
Rooney's goal - his eighth in six games for club and country - encapsulated why he is the most exciting of England's players.
Dimitar Berbatov sent Rooney racing away down the left with a glorious through ball from inside the centre circle.
Rooney raced clear before cutting inside Ryan Donk in the Albion box and scored with a cool low right-foot shot from 12 yards.
United's second goal 14 minutes later was another masterclass in passing.
Darren Fletcher's ball from the centre circle picked out Rooney, who threaded the perfect pass through for Cristiano Ronaldo to score with a low left-foot shot from 12 yards.
The third goal was United's softest. Nani's cross from the left edge of the Albion penalty area took a double deflection and landed perfectly for Berbatov to nonchalantly fire the ball home right-footed from eight yards on the far post.
With only seconds of the 90 minutes remaining, another superb passing move made it four for the home side.
Ryan Giggs' pinpoint pass found Berbatov on the right corner of the Albion box. He touched the ball on to Rooney to his right and his measured low cross to the far post was tapped home left footed by Nani from close range.
Rooney even had an effort disallowed by referee Mark Halsey in the 23rd minute.
Ronaldo set up the chance for Rooney who put the ball in the net with a sweet shot from 12 yards, only to see it disallowed for his push on Albion defender Gianni Zuiverloon.
Albion could have had few complaints about the result.
Goalkeeper Scott Carson made a string of fine saves to deny Rooney on three occasions and Berbatov once.
The Baggies only came to life as an attacking force after Ishmael Miller replaced Roman Bednar eight minutes into the second half.
Albion's best chance came a minute from time when Miller surged down the right and into the United box before unleashing a tremendous shot which was well saved by United goalkeeper Edwin van der Sar.