Triadelphia was originally chartered in 1829. Name adopted from the Greek word meaning three brothers, and probably named for the three sons of Colonel Josias Thompson, who donated the land upon which the town was originally laid out.
As of the census of 2000, there were 817 people, 370 households, and 223 families residing in the town. The population density was 457.2/km² (1,187.2/mi²). There were 406 housing units at an average density of 227.2/km² (590.0/mi²). The racial makeup of the town was 97.31% White, 1.59% African American, 0.12% Native American, and 0.98% from two or more races. Hispanic or Latino of any race were 0.73% of the population. 25.9% were of German, 13.4% Irish, 11.2% Italian, 9.3% Polish, 8.9% American and 7.0% English ancestry according to Census 2000.
There were 370 households out of which 24.6% had children under the age of 18 living with them, 44.9% were married couples living together, 11.1% had a female householder with no husband present, and 39.7% were non-families. 35.1% of all households were made up of individuals and 14.3% had someone living alone who was 65 years of age or older. The average household size was 2.21 and the average family size was 2.83.
In the town the population was spread out with 20.8% under the age of 18, 9.8% from 18 to 24, 29.7% from 25 to 44, 25.9% from 45 to 64, and 13.7% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 97.8 males. For every 100 females age 18 and over, there were 89.2 males.
The median income for a household in the town was $26,169, and the median income for a family was $36,136. Males had a median income of $25,441 versus $18,958 for females. The per capita income for the town was $14,267. About 15.8% of families and 15.8% of the population were below the poverty line, including 22.4% of those under age 18 and 6.0% of those age 65 or over.