Besides worldwide acclaim, the prizes for the winning dog and its owner are $1,500, a trophy, a photo session with photographer Kira Stackhouse, and VIP treatment at the Sheraton Sonoma County-Petaluma hotel that includes a catered dinner for the dog and the owner in the luxurious Loft Suite.
One of the judges this year is Jon Provost, who played "Timmy" in the Lassie television show.
Syndicated radio personalities Bob Lacey and Sheri Lynch, known as Bob and Sheri to listeners of The Mix 104.9 FM in Sonoma County, will host the contest.
A Dog Lovers' Festival starts at 1 p.m. and includes booths selling dog-friendly merchandise and the official 160-page World's Ugliest Dogs 25th Anniversary book, which contains humorous photos of the dogs taken by photographers, along with profiles of the dogs and their owners.
The four-legged contestants will walk the red carpet at 5:30 p.m., and the contest will begin on the Kiwanis stage at 6 p.m. "More than half the contestants are brand-new, and a lot of the time the winner is a new dog," Sonoma-Marin Fair spokeswoman Vicki DeArmon said.
"This year there are more pugs than ever," DeArmon said.
She expects 25-30 dogs will compete.
Other breeds in the contest are pedigrees and mutt mixes of Chihuahua, Chinese Crested, boxer, terrier, poodles and Peruvian. Their names include Sophie, Toots, Chupe, Cyndi Loo, Buster, Samson, Trixie, Monkey Man and Poloka.
Last year's winner was Mugly, a Chinese Crested dog from the United Kingdom. There are no entries from outside the United States this year.
The Sonoma-Marin Fair starts today at the Petaluma Fairgrounds and runs through Sunday.