Ontario is a city located in southwestern San Bernardino County, California, United States, east of downtown Los Angeles. Located in the western part of the Inland Empire region, it lies just east of the Los Angeles county line and is part of the Greater Los Angeles Area. As of the 2010 Census, the city had a population of 163,924, up from 158,007 at the 2000 census, making it the county's fourth most populous city after San Bernardino, Fontana, and Rancho Cucamonga.