San Mar is a census-designated place (CDP) in Washington County, Maryland, United States.

[5] According to the United States Census Bureau, the CDP has a total area of 1.3 square miles (3.4 km2), all land.

There were 178 housing units at an average density of 136.2 per square mile (52.6/km2).

28.7% of all households were made up of individuals, and 22.8% had someone living alone who was 65 years of age or older.

None of the families and 2.0% of the population were living below the poverty line, including no under eighteens and none of those over 64.