South River is a borough in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 16,008, reflecting an increase of 686 (+4.5%) from the 15,322 counted in the 2000 Census, which had in turn increased by 1,630 (+11.9%) from the 13,692 counted in the 1990 Census.

What is now South River was originally formed as the town of Washington within East Brunswick Township on February 23, 1870. South River was incorporated as an independent borough by an act of the New Jersey Legislature on February 28, 1898, replacing Washington town. It was named after the South River, which marks the borough's eastern and northeastern boundary.

South River was originally part of East Brunswick Township. Originally referred to as Washington, the community eventually split away as did Spotswood and Milltown.

According to the United States Census Bureau, the borough had a total area of 2.918 square miles (7.557 km²), including 2.769 square miles (7.171 km²) of land and 0.149 square miles (0.385 km²) of water (5.10%).

The borough borders the Middlesex County communities of East Brunswick Township and Sayreville.

Unincorporated communities, localities and place names located partially or completely within the borough include Newton Heights, which is situated along the border of South River and East Brunswick.

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