According to the United States Census Bureau, the town has a total area of 39.2 km² (15.1 mi²). 38.3 km² (14.8 mi²) of it is land and 0.9 km² (0.3 mi²) of it is water. The total area is 2.31% water. Plympton is 30 mi (47 km) south of Boston.
As of the census of 2000, there are 2,637 people, 854 households, and 737 families residing in the town. The population density is 68.8/km² (178.3/mi²). There are 872 housing units at an average density of 22.8/km² (59.0/mi²). The racial makeup of the town is 96.85% White, 0.99% African American, 0.61% Native American, 0.34% Asian, 0.00% Pacific Islander, 0.53% from other races, and 0.68% from two or more races. 0.42% of the population are Hispanic or Latino of any race.
There are 854 households out of which 43.3% have children under the age of 18 living with them, 76.5% are married couples living together, 7.0% have a female householder with no husband present, and 13.7% are non-families. 10.5% of all households are made up of individuals and 3.5% have someone living alone who is 65 years of age or older. The average household size is 3.09 and the average family size is 3.32.
In the town the population is spread out with 28.6% under the age of 18, 6.4% from 18 to 24, 29.5% from 25 to 44, 28.9% from 45 to 64, and 6.6% who are 65 years of age or older. The median age is 38 years. For every 100 females there are 96.6 males. For every 100 females age 18 and over, there are 91.3 males.
The median income for a household in the town is $70,045, and the median income for a family is $75,000. Males have a median income of $45,531 versus $34,000 for females. The per capita income for the town is $24,344. 2.1% of the population and 0.8% of families are below the poverty line. Out of the total people living in poverty, 1.7% are under the age of 18 and 3.4% are 65 or older.