Altmar is a hamlet (and census-designated place) in Oswego County, New York, United States. The population was 407 at the 2010 census.
Altmar is located at 43°30′43″N 76°0′3″W / 43.51194°N 76.00083°W / 43.51194; -76.00083 (43.511877, −76.000756).
According to the United States Census Bureau, the village has a total area of 2.2 square miles (5.7 km), of which, 2.2 square miles (5.7 km) of it is land and 0.46% is water.
As of the census of 2000, there were 351 people, 119 households, and 84 families residing in the village. The population density was 162.5 people per square mile (62.7/km²). There were 131 housing units at an average density of 60.6 per square mile (23.4/km²). The racial makeup of the village was 98.01% White, and 1.99% from two or more races.
There were 119 households out of which 31.1% had children under the age of 18 living with them, 52.1% were married couples living together, 12.6% had a female householder with no husband present, and 28.6% were non-families. 20.2% of all households were made up of individuals and 7.6% had someone living alone who was 65 years of age or older. The average household size was 2.88 and the average family size was 3.35.
In the village, the population was spread out with 29.3% under the age of 18, 12.3% from 18 to 24, 25.1% from 25 to 44, 21.9% from 45 to 64, and 11.4% who were 65 years of age or older. The median age was 32 years. For every 100 females, there were 93.9 males. For every 100 females age 18 and over, there were 89.3 males.
The median income for a household in the village was $31,786, and the median income for a family was $33,750. Males had a median income of $25,250 versus $21,250 for females. The per capita income for the village was $19,333. About 15.9% of families and 21.4% of the population were below the poverty line, including 28.7% of those under age 18 and 29.4% of those age 65 or over.