South Yarmouth is a census-designated place (CDP) in the town of Yarmouth in Barnstable County, Massachusetts, United States. The population was 11,603 at the 2000 census.
According to the United States Census Bureau, the CDP has a total area of 20.3 km² (7.8 mi²). 18.1 km² (7.0 mi²) of it is land and 2.2 km² (0.8 mi²) of it (10.70%) is water.
There were 5,485 households out of which 18.8% had children under the age of 18 living with them, 44.8% were married couples living together, 9.8% had a female householder with no husband present, and 42.0% were non-families. 36.3% of all households were made up of individuals and 22.3% had someone living alone who was 65 years of age or older. The average household size was 2.08 and the average family size was 2.67.
In the CDP the population was spread out with 17.1% under the age of 18, 4.7% from 18 to 24, 23.2% from 25 to 44, 22.6% from 45 to 64, and 32.4% who were 65 years of age or older. The median age was 49 years. For every 100 females there were 83.5 males. For every 100 females age 18 and over, there were 78.8 males.
The median income for a household in the CDP was $37,643, and the median income for a family was $44,325. Males had a median income of $35,476 versus $26,118 for females. The per capita income for the CDP was $20,609. About 5.5% of families and 7.3% of the population were below the poverty line, including 11.3% of those under age 18 and 3.9% of those age 65 or over.