The world champions from Brazil now take on Germany for the gold medal after third-seeded Julius Brink and Jonas Reckermann put out Dutch pair Reinder Nummerdor and Richard Schuil 21-14 21-16.
Emanuel, 39, is competing in his fifth Olympics and won medals in the last two. With former team-mate Ricardo Santos, he picked up gold in Athens eight years ago and bronze in Beijing in 2008.
Plavins and Smedins, who have never won a tournament together, enjoyed a terrific run of form at the London Games.
They upset Jake Gibb and Sean Rosenthal in the quarter-finals, ending U.S. participation in the men's event at an unusually early stage.
But the Latvians could not get the better of the agile Emanuel and the powerful Alison who have dominated the sport since they teamed up three years ago.
"When I started to play with Alison I was thinking I will play two more years and then think about quitting," Emanuel told reporters.
"But he put some fire inside me, he brought motivation again. Every day we train together I have something new to change my game. That's the good feeling I have about this final because I'm playing with a young player who gives me everything."
The Brazilians won the first set easily but were pushed harder in the second, the world champions needing two match points to finish off Plavins and Smedins.
The Latvians, who will play Nummerdor and Schuil in the bronze-medal match on Thursday, were pleased despite their defeat.
"It's a dream come true," said Plavins. "It's the best result ever for Latvia in beach volleyball.
"We are happy that in the second set we got 20 points. It's great. I hope Emanuel and Alison win the gold because they really deserve it."
Emanuel is a player of such standing in the world of beach volleyball that there is a statue of him at the entrance to the Olympic venue.
His former team-mate Ricardo also competed in the London Games but he and new partner Pedro Cunha were knocked out by Brink and Reckermann in the quarter-finals on Monday.
It was a low point in Ricardo's illustrious Olympic career.
The 37-year-old had competed in three previous Games and won medals each time - silver in 2000 with Ze Marco de Melo and gold and bronze in 2004 and 2008 with Emanuel.
But an even bigger surprise in London was the early exit of both American pairs, defending champions Todd Rogers and Phil Dalhausser and rivals Gibb and Rosenthal.
The United States had dominated since the sport made its Games debut in Atlanta. American teams won gold there in 1996 and again in Sydney in 2000 and in Beijing in 2008.
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