He beat German goalkeeper Oliver Kahn twice in 12 minutes for his seventh and eighth goals of the tournament.
The key moment was the first goal, when Kahn - voted the best keeper in the tournament - couldn't hold onto a shot from Rivaldo.
The ball fell to Ronaldo who slid the ball into the net to put Brazil in front.
Germany had started well in both halves, but they couldn't create any good chances, unlike Brazil.
The loss was harsh on Kahn who had been Germany's best player in the World Cup, but in the end Brazil were too good.