The competition offers a $10,000 grand prize for the winning pastry.
"It's no secret I love Tim Hortons," Priestley said in a release.
Priestley is best known for his role on the '90s television show "Beverly Hills, 90210."
In February he had a cameo on popular sitcom "How I Met Your Mother," in a role that inspired the contest.
The episode played on the idea of a Canada obsessed with Tim Hortons, where Priestley imagined an ideal doughnut. He was interviewed for a faux documentary where he bragged about a concoction called "The Priestley."
Following the episode, a Tim Hortons chef made a mock batch of what the envisioned pastry would be — a chocolate Timbit stuffed inside a strawberry-vanilla doughnut.
"My Tim Hortons dream doughnut became a reality, and now another lucky Tim Hortons fan could see theirs come to life too," Priestley said.
Since the competition's launch on Saturday, there have been more than 10,000 entries, said Tim Hortons spokeswoman Donna Finelli.
"We were so pleased with the opening weekend's results," she said. "These are Canadians who are really engaged with the brand and wanting to share their great ideas with us."
Tim Hortons is challenging the public to try to beat "The Priestley," Finelli said.
Monday's entries included the "9-0-2-1-Oreo," the chocolate "Hockey Puck" and the "Red Velvet Pecan" doughnut.
Though it's not certain whether the winning doughnut will be sold at Tim Hortons locations, Finelli said company chefs could use the idea for inspiration.
Head chef for research and development Wayne Blythe was the one who put "The Priestley" together.
"He's always combing the globe for newest trends on flavours and toppings and is really responsible for a lot of the innovation you see in our showcase," Finelli said.
Tim Hortons regularly adds new doughnuts to its menu and only two pastries, the Apple Fritter and the Dutchie, have been on the menu since the retailer's launch in 1964.
After Blythe made "The Priestley," Tim Hortons sent a photo of the creation out on Twitter, with the caption "A Timbit IN a doughnut? Genius."
The public's response to the Tweet made clear the interest in new doughnut flavours, Finelli said.
"There were all kinds of questions about whether 'The Priestley' would be available for sale," she said, adding that it was never made into a commercial reality. "But we thought we would give Canadians the opportunity to come up with their winning doughnut idea and that it would be perfect to have Jason as a judge."
Priestley and a panel of other judges will choose eight finalists from the entry pool, which closes July 21. The public then votes on a winner between Aug. 5 and 18.
Contest details and a doughnut design program are available on the company's website and Facebook page dubbed "Duelling Donuts."