The World's Ugliest Dog Contest is an annual contest held in Petaluma, California, as part of the Sonoma-Marin Fair, to decide which of the dogs entered in the contest is the ugliest. The contest, along with the rest of the fair, is typically scheduled for the fourth week of June. Along with the title of “The World’s Ugliest Dog” the winner’s owner receives a check for $1,000 and a trophy As of 2017, the prize has been increased to $1,600, a trophy and a free trip to New York City. There is significant media coverage, and as many as 20,000–30,000 people attend the contest during the fair each year. The fair now holds an all-day Dog Lovers' Festival preceding the evening contest.

The contest has been going on since the 1970s. In 2009 the Sonoma-Marin Fair trademarked the phrase World's Ugliest Dog. In 2013, the contest celebrated its 25th anniversary as a fair-produced event, and an anniversary book, World's Ugliest Dogs, was published with text by the contest's producer of seven years, Vicki DeArmon, and photos by photographers who have covered the contest over the years for the fair.