The Portuguese forward, whose every touch was greeted by cheers and piercing screams, scored one and made two other goals in a comprehensive victory for the English champions.
In front of a 65,000 sell-out crowd at the World Cup Stadium Seoul, languishing in mid-table in the K-League, were never a match for the Premiership giants and the English team cruised into a three-goal inside the opening half hour.
After five minutes Ronaldo, who appears to be building up a David Beckham-like status in the Far East, scored his second goal of the Asia Tour following neat inter-play with Rooney.
England striker Rooney received the ball from a throw-in halfway inside the Seoul half before turning and laying the ball off to the on-rushing Ronaldo.
With the defenders backing off, Ronaldo powered forward before beating keeper Kim Byung-Ji with a low shot from 20 yards.
Ki Sung Yeung gave United a fright seven minutes later when he shot narrowly wide from the edge of the penalty area, but on 18 minutes, Sir Alex Ferguson's team had doubled their lead through youngster Chris Eagles.
Ronaldo was again a key figure as he played tricks with the ball before teeing up the young midfielder and Eagles beat Kim with a curling effort into the bottom corner of the net.
The home side simply could not compete with United's superior quality and Rooney put United 3-0 ahead on 20 minutes with well-taken goal, again after being teed up by Ronaldo.
Ronaldo almost made it 4-0 on 29 minutes with a vicious left-foot strike from 30 yards that flew inches over the crossbar.
Rooney and Ronaldo were replaced at half-time by Ryan Giggs and Alan Smith and Seoul were able to settle quicker after the break, creating chances to reduce the deficit.
But Kim Dong Suk's low effort was saved by goalkeeper Tomasz Kuszczak and United unsurprisingly regained the upper hand shortly afterwards, with Senegal-born France defender Patrice Evra rounding off a two-man move involving Giggs and Paul Scholes to score from close range after 59.