Fontana is a city in San Bernardino County, California, United States. Founded by Azariel Blanchard Miller in 1913, it remained essentially rural until World War II, when entrepreneur Henry J. Kaiser built a large steel mill in the area. It is now a regional hub of the trucking industry, with the east–west Interstate 10 and State Route 210 crossing the city and Interstate 15 passing diagonally through its northwestern quadrant. The city is about 46 miles (74 km) east of Los Angeles.

The United States Census Bureau reported that Fontana's 2020 population was 208,393, making it the second-most-populous city in San Bernardino County and the 20th largest in the state.

Native Americans inhabited the area.

Fontana, formerly Rosena from 1890 to 1919, was founded in 1919 by Azariel Blanchard Miller. The name fontana is Italian for fountain or water source; the city is close to the Santa Ana River to the east. Within a few years, it became an agricultural town of citrus orchards, vineyards and chicken ranches and astride U.S. Route 66 (now known as Foothill Boulevard). The Fontana area was radically transformed during World War II when Henry J. Kaiser built the Kaiser Steel plant just outside the city limits. At the time, it was one of only two steel mills west of the Mississippi River. To provide for the plant workers' health needs, Henry J. Kaiser constructed the Fontana Kaiser Permanente medical facility, now the largest managed care organization in the United States.

In the 1950s and 1960s, Fontana was home to a drag racing strip that was a venue in the NHRA circuit. Mickey Thompson's Fontana International Dragway was also referred to as Fontana Drag City or Fontana Drag Strip. The original Fontana strip is gone, but the owners of NASCAR's new Auto Club Speedway opened a NHRA-sanctioned drag strip just oustside Fontana in mid-2006.

Ro-Val's automobile museum, located on Foothill Boulevard on the western outskirts between Fontana and Cucamonga, was the home for many classic automobiles of the 1920s and 1930s, including a huge vehicle once owned by screen actor Fatty Arbuckle. When the Ro-Val museum closed, the vehicles were sold to Bill Harrah, a Nevada casino owner and automobile collector, who placed them on display in the museum located at his casino.

In 2000, the city had a total population of 128,929; by 2020, the city had 212,704 residents. This rapid growth was largely due to the numerous large, new residential developments built in the sparsely populated northern part of the city, as well as with the city's aggressive (and highly successful) campaign to annex several unincorporated, but developed, San Bernardino County areas in 2006–2007.

In 2019, the California Air Resources Board advised the City against housing people within 1,000 feet of industrial warehouses because of harmful truck pollution. The city was also sued by San Bernardino County, the Center for Biological Diversity, the Sierra Club and the Center for Community Action and Environmental Justice over the approval of West Valley Logistics Center, which violated state environmental laws.