Diamond Bar, CA has a population of 56,456 and is the 705th largest city in the United States. The population density is 3,794 per sq mi which is 1533% higher than the California average and 4088% higher than the national average. The median age in Diamond Bar is 42 which is approximately 17% higher than the California average of 36. In Diamond Bar, 59% of the population over 15 years of age are married, 42% speak English and 10% speak Spanish. 45% of Diamond Bar residents were born in California, 10% were born out of state, 1% were born outside of the United States and 43% were foreign born.