Diamond Bar is an upper middle class city in eastern Los Angeles County, California, United States. The population was 55,544 at the 2010 census, down from 56,287 at the 2000 census. It is named after the "diamond over a bar" branding iron registered in 1918 by ranch owner Frederick E. Lewis.

Located at the junction of the Pomona and Orange freeways, Diamond Bar is primarily residential with shopping centers interspersed throughout the city. It is surrounded by the suburban cities, such as Brea, Walnut, Chino Hills, and Rowland Heights. The city features a public Los Angeles County golf course.

Diamond Bar, CA