Marblemount is located at (48.538154, -121.437844)
According to the United States Census Bureau, the CDP has a total area of 2.5 square miles (6.4 km²), of which, 2.4 square miles (6.1 km²) of it is land and 0.1 square miles (0.3 km²) of it (4.44%) is water.