Cormier (9-0) knocked out Brazil's Antonio (Bigfoot) Silva (16-3) while Barnett (31-5) choked out Russia's Sergei Kharitonov (18-5).
The final will take place in early 2012.
"I think Cormier's a great opponent. He's a guy that's competed at the highest level of athletics," Barnett said of the two-time Olympic wrestler. "I know I've got to train real hard to fight him."
Earlier at the U.S. Bank Arena, Luke Rockhold dethroned Strikeforce middleweight champion Ronaldo (Jacare) Souza, winning a unanimous 50-45, 48-47, 48-47 decision.
Fedor Emelianenko, Andrei Arlovski, Brett (The Grim) Rogers and Fabrico Werdum lost earlier this year in the heavyweight tournament.
Strikeforce heavyweight champion Alistair Overeem never made it to the semis. The Big Dutchman was cut, apparently over a scheduling dispute, only to sign with the UFC, which owns Strikeforce.
Cormier came in as Overeem's replacement in the tournament. At 5-11 and 249 pounds, he was giving up five inches in height, 11 in reach and 16 pounds to Silva.
But Cormier was faster and he used that advantage
Cormier knocked Silva down with a right in the first minute, then beckoned the big Brazilian to his feet. Silva ended on his back two more times in the round before Cormier felled him with a left hook and right uppercut and finished him with a hammer fist at 3:56.
"I fought one of the top five heavyweights in the world and I knocked him out," said Cormier, who trains with UFC heavyweight champion Cain Velasquez at American Kickboxing Academy in San Jose. "Give me some respect now."
Cormier said he may have injured his hand in the fight. "Let's hope it's not anything bad," he said.
Barnett, formerly known as The Baby-Faced Assassin, was also rampant in his win.
Kharitonov connected with some punches early on before Barnett took him down via a trip and quickly got in mount position. The Russian looked to ride out the round as Barnett punched from above but eventually gave up his back and turtled.
Barnett slickly shifted position and locked in an arm-triangle side choke that forced the Russian to tap at 4:28.
The six-foot-three, 261-pound Barnett is a former Pride veteran and UFC heavyweight champion. Kharitonov, 6-3 and 256, also fought in Pride.
Rockhold, whose eight previous bouts all ended in the first round, had not fought for 19 months due to injury. But he looked bright this night.
Rockhold (8-1) was the busier fighter in a gruelling five-round fight contested almost entirely on the feet. Brazil's Souza (14-3-1) had more power but seemed to run out of steam and was unable to get the fight to the ground.
Two Canadians won on the undercard.
Welterweight Jordan (Young Gun) Mein won his Strikeforce debut, stopping veteran Brazilian Evangelista (Cyborg) Santos in the third round.
The 21-year-old from Lethbridge, Alta., tagged Santos (18-15) several times in the final round, bloodying his face. Mein (23-7) then drove him backwards with a punch to the liver and left to the head, finishing him off with a dozen elbow strikes at the fence at 3:18 of the round.
In women's action, bantamweight Alexis Davis (11-4) of Port Colbourne, Ont. stopped Amanda (Lioness of the Ring) Nunes (6-2) at 4:53 of the second round.
Lightweight Pat (Bam Bam) Healy (27-16), stepping in for the injured Josh Thompson, choked out Venezuelan Maximo Blanco (8-3-1 with one no contest) at 4:24 of the second round.
Former light-heavyweight title-holder Muhammed (King Mo) Lawal (8-1) knocked out Roger Gracie (4-1) at 4:33 of the first round.
In other undercard action, former light-heavyweight champion Rafael (Feijao) Cavalcante knocked out Yoel Romero at 4:51 of the second round.
Romero, who won Olympic silver in 2000 in wrestling for Cuba, was making his Strikeforce debut.
Cavalcante took the 205-pound title from Lawal, only to lose it to Dan Henderson.
Veteran light-heavyweight Mike Kyle (19-8-1 with one no contest) won a unanimous decision over light-heavyweight Marcos Rogerio de Lima (9-1).
Middleweight Dominique Steele (4-1) won a unanimous decision over Chris Mierzwiak (3-1).