As of 2000, there were 12,386 people, 5,471 households, and 3,545 families residing in the town. The population density was 227.3/km² (588.6/mi²). There were 9,450 housing units at an average density of 173.4/km² (449.1/mi²). The racial makeup of the town was 95.41% White, 0.71% Black or African American, 0.19% Native American, 0.22% Asian, 0.05% Pacific Islander, 2.03% from other races, and 1.40% from two or more races. 0.96% of the population were Hispanic or Latino of any race.

There were 5,471 households out of which 21.3% had children under the age of 18 living with them, 53.4% were married couples living together, 9.0% had a female householder with no husband present, and 35.2% were non-families. 29.8% of all households were made up of individuals and 16.9% had someone living alone who was 65 years of age or older. The average household size was 2.20 and the average family size was 2.72.

In Harwich the population was spread out with 18.3% under the age of 18, 4.2% from 18 to 24, 22.1% from 25 to 44, 25.8% from 45 to 64, and 29.6% who were 65 years of age or older. The median age was 49 years. For every 100 females there were 84.5 males. For every 100 females age 18 and over, there were 79.7 males.

The median income for a household in Harwich was $41,552, and the median income for a family was $51,070. Males had a median income of $38,948 versus $27,439 for females. The per capita income for the town was $23,063. About 2.9% of families and 5.5% of the population were below the poverty line, including 8.4% of those under age 18 and 4.1% of those age 65 or over.

In the summer, the town is host to the Harwich Mariners of the Cape Cod Baseball League. The team plays at Whitehouse Field.