Ontario is the largest city in Malheur County, Oregon, United States. It lies along the Snake River at the Idaho border. The population was 11,645 at the 2020 census. The city is the largest community in the region of far eastern Oregon, also known as the Western Treasure Valley.

Malheur County is one of the 36 counties in the U.S. state of Oregon. As of the 2020 census, the population was 31,571. Its county seat is Vale, and its largest city is Ontario. The county was named after the Malheur River, which runs through the county. The word "malheur" is French for misfortune or tragedy. Malheur County is included in the Ontario, Oregon Micropolitan Statistical Area, which is also included in the Boise Combined Statistical Area. It is included in the eight-county definition of Eastern Oregon.