Fullerton (/ˈfʊlərtən/ FULL-er-tun) is a city in northern Orange County, California, United States. As of the 2020 census, the city had a total population of 143,617.

Fullerton was founded in 1887. It secured the land on behalf of the Atchison, Topeka and Santa Fe Railway. Historically it was a center of agriculture, notably groves of Valencia oranges and other citrus crops; petroleum extraction; transportation; and manufacturing. It is home to numerous higher educational institutions, particularly California State University, Fullerton and Fullerton College. From the mid-1940s through the late 1990s, Fullerton was home to a large industrial base made up of aerospace contractors, canneries, paper products manufacturers, and is considered to be the birthplace of the electric guitar, due in large part to Leo Fender. The headquarters of Vons, which is owned by Albertsons, is located in Fullerton near the Fullerton–Anaheim line.

Evidence of prehistoric animal habitation, such as saber-toothed cats and mammoths, is present in Ralph B. Clark Regional Park in the northwest of the city.

The area of the city was a part of the homelands of the Tongva for thousands of years. There was a large village in the area along the Santa Ana River that has since been identified as the Hutuknga. The village was one of the largest throughout all of Tovaangar, or the Tongva world. It was connected by marriage ties to other villages in the area, including Genga. Acorns and seeds from grasses and sages were regularly cultivated. Trade connections were established with villages on the coast and those further inland.

Europeans first passed through the area in 1769 when Gaspar de Portolá led a Spanish expedition north to Monterey. From the description recorded in the diary of Father Juan Crespi, the party camped on July 29 near present-day Laguna Lake, in the Sunny Hills area.

In 1894, Charles Chapman purchased an orange orchard in eastern Fullerton. The Valencia variety of oranges he promoted from his Santa Ysabel Ranch, well suited to the local climate, proved a boon to producers; Fullerton boasted more orange groves than any other municipality in the United States. Cultivation of walnuts and avocados also flourished, and the Western railroad town became an agricultural center. Fullerton was incorporated in 1904,

Drilling for petroleum began in 1880 with the discovery of the Brea-Olinda Oil Field and fueled the first real boom, peaking in the 1920s. Construction reflected the vogue for Spanish Colonial and Italian Renaissance–inspired architecture, as in the historic Fox Fullerton Theatre (erected 1925); the Muckenthaler House, designed by Frank K. Benchley (erected 1924); and the city's chief landmark, the Plummer Auditorium and clock tower (erected 1930). Fullerton College was established at its present location at Chapman Avenue and Lemon Street in 1913. Meanwhile, the city banned all overnight street parking in 1924 – a law enforced to the present day, unless an area is specifically exempted.

In 1943, the Alex Bernal residence became the site of one of the first successful lawsuits against household covenant laws in the country after Alex and Esther Bernal purchased a home in a Fullerton neighborhood that barred purchases from "Mexicans." After a home invasion that resulted in their belongings being thrown into the street and a petition signed by fifty neighborhood residents to have the Bernal's removed from the neighborhood, a lawsuit was issued against the Bernals on the basis that their presence caused "irreparable injury" to the neighborhood that could lead to "coming in contact with said other races, including Mexicans, in a social and neighborhood manner." Lawyer David C. Marcus represented the Bernals in Doss vs. Bernal and won the case, which received national attention.