South Pasadena is a city in Los Angeles County, California, United States. As of the 2010 census, it had a population of 25,619, up from 24,292 at the 2000 census. It is located in the West San Gabriel Valley. It is 3.42 square miles in area and lies between the much larger City of Pasadena, of which it was once a part, and the metropolis of Los Angeles.