South El Monte is a city in the San Gabriel Valley, in Los Angeles County, California, United States. At the 2020 census, the city had a population of 19,567, down from 20,116 at the 2010 census.

According to the United States Census Bureau, the city has a total area of 2.8 square miles (7.4 km²), virtually all land.

South El Monte first appeared as a city in the 1960 U.S. census as part of the Upper San Gabriel Valley census county division.

The 2020 United States census reported that South El Monte had a population of 19,567. The population density was 6,880.1 inhabitants per square mile (2,656.4/km²). The racial makeup of South El Monte was 13.4% White, 0.5% African American, 2.7% Native American, 17.4% Asian, 0.0% Pacific Islander, 45.5% from other races, and 20.6% from two or more races. Hispanic or Latino of any race were 79.6% of the population.

The census reported that 99.6% of the population lived in households, 0.3% lived in non-institutionalized group quarters, and 0.1% were institutionalized.

There were 4,921 households, out of which 47.9% included children under the age of 18, 49.0% were married-couple households, 7.3% were cohabiting couple households, 25.3% had a female householder with no partner present, and 18.5% had a male householder with no partner present. 11.0% of households were one person, and 4.9% were one person aged 65 or older. The average household size was 3.96. There were 4,103 families (83.4% of all households).

The age distribution was 23.7% under the age of 18, 10.3% aged 18 to 24, 28.1% aged 25 to 44, 25.0% aged 45 to 64, and 12.9% who were 65 years of age or older. The median age was 35.6 years. For every 100 females, there were 100.7 males.

There were 5,058 housing units at an average density of 1,778.5 units per square mile (686.7 units/km²), of which 4,921 (97.3%) were occupied. Of these, 46.6% were owner-occupied, and 53.4% were occupied by renters.