Diamond Bar is a city in eastern Los Angeles County, California, United States. As of the 2010 census it had a population of 55,544, and in 2018 the population was estimated to be 56,275. It is named after the “diamond over a bar” branding iron registered in 1918 by ranch owner Frederick E. Lewis. The city features a public Los Angeles County golf course. It is also home to the Diamond Bar Country Estates, a private guarded community.