Rosemead is a city in Los Angeles County, California, United States. The 2020 United States Census reported a population of 51,185. Roseme…Rosemead is a city in Los Angeles County, California, United States. The 2020 United States Census reported a population of 51,185. Rosemead is part of a cluster of cities, along with Alhambra, Arcadia, Temple City, Monterey Park, San Marino, and San Gabriel, in the west San Gabriel Valley with a growing Asian-American population.