According to the United States Census Bureau, Old Tappan borough had a total area of 4.197 square miles (10.870 km2), including 3.332 square miles (8.629 km2) of land and 0.865 square miles (2.241 km2) of water (20.62%)
The borough is bordered to the north by the hamlets of Pearl River and Tappan in the town of Orangetown in Rockland County, New York.