Barcelona struggled to reproduce the same form that they showed in their 4-0 demolition of Mexico's America 4-0 in the semi-finals.
But Inter, who scraped past Egypt's Al-Ahli 2-1 to reach the final, rarely threatened and Adriano's breakaway winner was a bolt from the blue from the Libertadores Cup holders.
"I'm over the moon," said Adriano. "It's great to beat Barcelona. We were under a lot of pressure in the first half but we got better as the game went on."
Iarley, who made the goal when he robbed Barca captain Carlos Puyol near halfway, said Inter had reaped the rewards from their patient play.
"The match is for 90 minutes. We just waited for our opportunity," he said.Chances squandered
Ronaldinho, one of Barcelona's Brazilians, squandered the best chance of the first half, blasting into the side-netting after Clemer had spilled Deco's long-range shot in the 19th minute.
|Super sub Adriano, centre, celebrates|
In the second, Inter showed more adventure but Barcelona substitute Xavi was looking dangerous, volleying at Clemer after almost picking out Gudjohnsen twice.
However, it was the Inter substitute that made the difference, flicking the ball right-footed past goalkeeper Victor Valdes.
Deco was voted player of the tournament but it was scant consolation for a Barcelona side bidding to go one better than Johan Cruyff's stylish side of the early 1990s.
Cruyff's team lost 2-1 to Sao Paulo in 1992 in the Club World Cup's forerunner - a one-off game between the European and South American champions.
Inter's victory made it two wins out of two for Brazilian teams in the revamped Fifa tournament involving the world's six continental champions. Sao Paulo beat Liverpool 1-0 in last year's final. Al-Ahly finish third
Earlier, Mohamed Aboutrika scored two goals to give Egypt's Al-Ahly a 2-1 win over Mexico's Club America and secure third place in the tournament.
Aboutrika scored the winner in the 79th minute after completing a superb give-and-go with Flavio, rifling a low shot past America goalkeeper Guillermo Ochoa.
It was an impressive performance for the African champions, who finished in last place at the 2005 tournament.
"This is a good result for us," said Aboutrika. "After finishing last a year ago, it was satisfying to score twice and help our team finish third."