The visitors battled hard early on while Rooney had an effort disallowed. But the in-form England international cut inside Ryan Donk to fire a superb opener on 55 minutes before setting up Cristiano Ronaldo for a simple second.
Dimitar Bebatov added a casual third after poor Baggies defending before Rooney set up Nani to slide in late on.
Ronaldo slipped a neat pass in for Rooney who muscled in front of a defender and delayed his shot before scooping the ball past Scott Carson.
United manager Sir Alex Ferguson had to make a change after 35 minutes, bringing on John O'Shea for Patrice Evra, The Frenchman seemingly having collected a knock.
Dimitar berbatov made no mistake to score when the breakthough finally came in the 55th minute.
Ferguson sent Gary Neville on for Rafael Da Silva in the 65th minute with the young brazilian fading after a strong first half.
Berbatov had made his first goal in this Premier League.
Van der Sar, R. Da Silva (Neville 65), Ferdinand, Vidic, Evra (O'Shea 36), Ronaldo, Fletcher, Giggs, Park (Nani 70), Berbatov, Rooney.
Subs not used: Kuszczak, Brown, Evans, Gibson.
Goals: Rooney 56, Ronaldo 69, Berbatov 71, Nani 90.
Referee: Mark Halsey
Man-of-the-Match: Wayne Rooney
Source: BBC Sports.
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