South Lancaster is a census-designated place (CDP) in the town of Lancaster and close to the Town of Clinton in Worcester County, Massachusetts, United States. The population was 1,642 at the 2020 census.

South Lancaster is located at 42°26′16″N 71°41′18″W / 42.43778°N 71.68833°W / 42.43778; -71.68833 (42.437841, -71.688444).

According to the United States Census Bureau, the CDP has a total area of 3.4 km² (1.3 mi²), of which 3.4 km² (1.3 mi²) is land and 0.76% is water.

As of the 2020 census, South Lancaster had a population of 1,838. The median age was 42.6 years. 22.0% of residents were under the age of 18 and 18.3% of residents were 65 years of age or older. For every 100 females there were 91.3 males, and for every 100 females age 18 and over there were 86.1 males age 18 and over.

100.0% of residents lived in urban areas, while 0.0% lived in rural areas.

There were 735 households in South Lancaster, of which 31.8% had children under the age of 18 living in them. Of all households, 49.4% were married-couple households, 15.2% were households with a male householder and no spouse or partner present, and 28.7% were households with a female householder and no spouse or partner present. About 29.6% of all households were made up of individuals and 13.8% had someone living alone who was 65 years of age or older.

There were 799 housing units, of which 8.0% were vacant. The homeowner vacancy rate was 1.4% and the rental vacancy rate was 5.0%.

As of the census of 2000, there were 1,742 people, 676 households, and 469 families residing in the CDP. The population density was 517.4/km² (1,334.9/mi²), amongst the lowest in the state. There were 695 housing units at an average density of 206.4/km² (532.6/mi²). The racial makeup of the CDP was 90.30% White, 6.43% African American, 0.06% Native American, 1.26% Asian, 1.49% from other races, and 0.46% from two or more races. Hispanic or Latino of any race were 5.45% of the population.