Dennis Port is located at 41°39′50″N 70°8′12″W / 41.66389°N 70.13667°W / 41.66389; -70.13667 (41.663779, -70.136576).

According to the United States Census Bureau, the CDP has a total area of 8.5 square kilometres (3.3 sq mi). 3.0 square miles (7.8 km²) of it is land and 0.27 square miles (0.7 km²) of it (7.72%) is water.

Dennis Port is said by locals to be a part of the Lower Cape Cod region unlike the rest of Dennis villages.^{[citation needed]}

As of the census of 2000, there were 3,612 people, 1,719 households, and 943 families residing in the CDP. The population density was 455.8/km² (1,181.3/mi²). There were 4,369 housing units at an average density of 551.3/km² (1,428.9/mi²). The racial makeup of the CDP was 90.03% White, 4.90% African American, 0.36% Native American, 0.39% Asian, 1.74% from other races, and 2.57% from two or more races. Hispanic or Latino of any race were 2.99% of the population.

There were 1,719 households, out of which 20.4% had children under the age of 18 living with them, 39.6% were married couples living together, 11.9% had a female householder with no husband present, and 45.1% were non-families. 37.9% of all households were made up of individuals, and 16.8% had someone living alone who was 65 years of age or older. The average household size was 2.06 and the average family size was 2.68.

In the CDP, the population was spread out, with 17.9% under the age of 18, 6.2% from 18 to 24, 28.1% from 25 to 44, 24.2% from 45 to 64, and 23.6% who were 65 years of age or older. The median age was 43 years. For every 100 females, there were 87.3 males. For every 100 females age 18 and over, there were 84.2 males.

The median income for a household in the CDP was $36,518, and the median income for a family was $41,525. Males had a median income of $36,568 versus $29,398 for females. The per capita income for the CDP was $23,872. About 10.5% of families and 10.7% of the population were below the poverty line, including 19.6% of those under age 18 and 6.1% of those age 65 or over.