Old Tappan (English pronunciation: /tæˈpæːn/ ^ⓘ ) is a borough in northern Bergen County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 5,888, an increase of 138 (+2.4%) from the 2010 census count of 5,750, which in turn reflected an increase of 268 (+4.9%) from the 5,482 counted in the 2000 census.

Old Tappan was incorporated as a borough by an act of the New Jersey Legislature on October 18, 1894, from portions of Harrington Township, based on the results of a referendum held two days earlier. The borough was formed during the "Boroughitis" phenomenon then sweeping through Bergen County, in which 26 boroughs were formed in the county in 1894 alone. On April 23, 1896, additional territory was annexed from Harrington Township. The borough's name is derived from the Tappan tribe of Native Americans.

According to the United States Census Bureau, the borough had a total area of 4.163 square miles (10.78 km²), including 3.315 square miles (8.59 km²) of land and 0.848 square miles (2.20 km²) of water (20.38%).

One of only four confluence points in New Jersey, the 41°N 74°W crossing, is in Old Tappan on watershed property owned by United Water.

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. Lake Tappan and the Hackensack River are on the western side of the town, bordering River Vale. Harrington Park is to the south and Northvale and Norwood are to the east.

As of 2026, the borough is a member of Local Leaders for Responsible Planning in order to address the borough's Mount Laurel doctrine-based housing obligations.

As of the 2020 census, the borough had a population of 5,888. The median age was 46.6 years. Of residents, 22.7% were under age 18 and 19.9% were age 65 or older. For every 100 females, there were 96.9 males, and for every 100 females age 18 and over, there were 92.3 males.

All residents were counted in urban areas (100.0%), with 0.0% in rural areas.