Ontario is a city located in San Bernardino County, California, United States. As of the 2000 Census, the city had a total population of 170,373, making it the 18th largest city in Southern California. The city is home to the LA/Ontario International Airport which is the 15th busiest cargo airport in the United States. Ontario handles the mass freight traffic from the ports of Los Angeles and Long Beach, and the rest of the country. It is also the home of Ontario Mills and former home of the Ontario Motor Speedway.

It takes its name from the Ontario Model Colony development established in 1882 by the Canadian engineer George Chaffey and his brothers William Chaffey and Charles Chaffey. They named the settlement after their home province of Ontario, Canada.