Messi's Argentina routed Brazil 3-0 to reach its second straight Olympic final, putting an end to Ronaldinho's hopes of winning his nation's first gold medal in football.
All eyes were on the former FC Barcelona teammates, the tournament's top stars.
Ronaldinho failed to come through for the Brazilians, while Messi was key for the Argentine victory, setting up two goals.
"We all know Messi's quality," Brazil coach Dunga said. "Sometimes you can mark him, but sometimes you can't. It's what happens with players with great individual quality."
In his first Olympics, Messi started the play on the team's second goal by eluding several defenders before feeding the ball on the right to Ezequiel Garay, who found Sergio Aguero for his close-range goal in the 58th minute.
Messi also set up Aguero inside the area before he was fouled in the play that prompted the third goal on a penalty kick, converted by Juan Roman Riquelme in the 76th. Aguero had opened the scoring from inside the area in the 52nd.
"It feels really good to be in the final," Messi said. "I didn't image we could score like this. Now we are ready for gold."
The Argentine also had chances to score himself. He cleared three defenders inside the area in the 41st before firing a shot that deflected on a defender and was barely saved by Brazil goalkeeper Renan.
In the 32nd, Messi sent a dangerous shot across the area but the Argentine forwards failed to connect.
Messi was one of the Argentine players celebrating inside the team's bus as it was set to leave Workers' Stadium, jumping around and chanting with his teammates.
Ronaldinho's best chance came with a free kick in the 65th, when his right-footed shot struck the post with goalkeeper Sergio Romero already beaten. Ronaldinho's AC Milan teammate, Alexandre Pato, scored off the rebound on the free kick, but the goal was disallowed because of offside.
"It was normal (Ronaldinho) couldn't play in these Olympics at the level everyone is used to seeing from him," Dunga said. "Without a doubt he was an example for the team on and off the field. He will still contribute a lot for the national team."
The 28-year-old Ronaldinho, who struggled with injuries at Barcelona, got to play only because he negotiated his release before transferring to AC Milan.
Countries are allowed to include only three players older than 23 in the Olympics.
Messi was allowed to participate even though Barcelona could have requested the 21-year-old playmaker to return after a court ruling just a day before the tournament was to begin.
Each player had scored twice in the tournament, with Ronaldinho netting a pair of goals in Brazil's 5-0 routing of New Zealand in the group stage. Messi scored once in Argentina's 2-1 win over the Netherlands in the quarterfinals and another in the 2-1 victory over the Ivory Coast in the group stage.
Ronaldinho played all minutes for Brazil, while Messi was rested in Argentina's 2-0 win over Serbia, when the Argentines had already secured a quarterfinal berth.
Argentina, trying to become the first nation to win consecutive titles in 40 years, will now face Nigeria in a rematch of the 1996 final in Atlanta, when the Africans won 3-2 for their first and only title.
Tuesday's victory was the first for Argentina against Brazil in three years.
