Sayreville is a borough in Middlesex County, in the U.S. state of New Jersey. Sayreville is within the heart of the Raritan Valley region, located on the south banks of the Raritan River, and also located on the Raritan Bay. As of the 2020 United States census, the borough's population was 45,345, an increase of 2,641 (+6.2%) from the 2010 census count of 42,704, which in turn reflected an increase of 2,327 (+5.8%) from the 40,377 counted in the 2000 census.

The Raritan River is a major river of New Jersey. Its watershed drains much of the mountainous area of the central part of the state, emptying into the Raritan Bay on the Atlantic Ocean.