Fifth seed Rublev, who won the Madrid Masters title in May, overcame a two-hour rain interruption as he beat Italy's Matteo Arnaldi 6-4, 6-2 in their semi-final.
He faces off Monday evening for the trophy against Australian Popyrin, who put out last week's ATP Washington winner Sebastian Korda 7-6, 6-3 in 92 minutes.
Rublev has had a patchy 2024, melting down several times on court in frustration with his play and being disqualified last February in Dubai for blowing up at a linesman.
But the Russian, who skipped the Paris Olympics to be prepared for the North American hardcourts that continue next week in Cincinnati, said he made the right scheduling choice.
"That was the plan, to be set for the US Open," he said. "I guess if I'm in the finals of Montreal, it must be working."
Rublev will be competing in the sixth Masters final of his career and tuned up for the title bid by knocking out top-seeded world number one Jannik Sinner in a Saturday quarter-final.
He said it was not easy facing Arnaldi, whose ranking will move to around 30th, putting him in the frame for a US Open seeding.
"He can hit hard out of nowhere and doesn't give you any rhythm. The wind did not help," Rublev said.
"After the rain delay I played much better. I was focused and could dictate and play more aggressive."
Popyrin twice traded first-set breaks with Korda, but totally dominated the tiebreaker to love.
The Aussie broke to start the second set and held the margin all the way to victory as he reached his first Masters final.
"It means the world to reach this final, it's amazing," he said. "It's an amazing feeling and achievement. I have to pat myself on the back.
"But tomorrow will be all hands on deck and back to work.
"The first set was topsy-turvy, a few breaks here and there. But I stayed mentally strong and took momentum into the tiebreaker.
"I really needed to win in two sets - I don't know if my legs would have made it into a third."
'He's really quick'
Popyrin beat Rublev on clay last spring in Monte Carlo but knows the seed is also a huge hardcourt danger.
"He plays a tough game on hardcourt," Rublev said. "He's really quick."
Rublev came out after the rain interruption in his win over Arnaldi and produced a double break after less than 30 minutes to reach the final.
"It was worth it, all this week was worth it," Rublev said of the rain delays that have plagued the event.
"I'm happy to be in my first Canadian final. Now I just want to recover, rest and be ready for tomorrow."
In a pair of quarter-finals held over by rain, Alexander Zverev missed a chance to collect his 50th victory of the season in a 7-6, 1-6, 6-4 loss to American Korda and Popyrin accounted for Hubert Hurkacz 3-6, 7-6, 7-5.
