Diamond Bar is a city in eastern Los Angeles County, California, United States. At the 2010 census it had a population of 55,544, and in 2019 the population was estimated to be 55,720. It is named after the "diamond over a bar" branding iron registered in 1918 by ranch owner Frederic E. Lewis (1884–1963). The city features a public Los Angeles County golf course.

Rancho San Jose was a 22,340-acre (90.4 km2) Mexican land grant in northeastern Los Angeles County given in 1837 by Governor Juan Bautista Alvarado to Ygnacio Palomares and Ricardo Véjar. Today, the communities of Pomona, LaVerne, San Dimas, Diamond Bar, Azusa, Covina, Walnut, Glendora, and Claremont are located in whole or part on land that was once part of the Rancho San Jose.