Azusa is a city in the San Gabriel Valley, at the foot of the San Gabriel Mountains in Los Angeles County, California, United States.The A on the San Gabriel Mountains represents the city of Azusa, and can be seen within a 30-mile radius. The population was 46,361 at the 2010 census, up from 44,712 at the 2000 census. Azusa is located along historic Route 66, which passes through the city on Foothill Boulevard and Alosta Avenue.