The match in Rome had been hyped as a duel between the world's two best players, Argentine Messi and United's Portuguese winger Cristiano Ronaldo.
Ronaldo was largely ineffective apart from an early flourish and it was Messi who came out on top, scoring his ninth goal of the Champions League campaign and improving his chances of snatching away his rival's FIFA World Player of the Year crown.
"This is the most important win of my life," Messi told reporters. "I am the happiest man alive and I am happy for myself, for my family and for my country."
Barca coach Pep Guardiola said Messi had been superb for the Primera Liga champions all season, helping not only with goals but also with plenty of assists.
"He has done great," he told a news conference. "I think he is the best player and we didn't need to win to know he is the best."
It might have been different if a dipping Ronaldo freekick in the second minute had slipped through the arms of Barca goalkeeper Victor Valdes instead of rebounding away from goal.
As well as the freekick, he had a couple of early shots but then drifted out of the match and was booked late on for an unnecessary barge on Barca captain Carles Puyol.
FORLORN FIGURE
He cut a forlorn figure trudging up last to collect his runners-up medal and was loudly whistled by the Barca fans.
"I don't have any problems with anyone and I never will. It was just a game situation," Ronaldo said of the Puyol incident.
"After the first goal things got complicated and we couldn't play our game," he told reporters.
United manager Alex Ferguson said Guardiola's decision to move Messi further back into midfield had made it hard for his side to get back into the game after conceding the opening goal in the 10th minute.
Messi made 51 passes on the night with a success rate of 84 percent, while Ronaldo made 31 and completed 68 percent.
"Messi moving into midfield made it difficult to get the ball back off them," he told a news conference.
"It was a disappointing performance and some individuals will feel that themselves."
