The Matanzas River is a body of water in St. Johns and Flagler counties in the U.S. state of Florida.

The waterbody connects ocean inlets in the Port of St. Augustine and at the Fort Matanzas National Monument.

[3] The Matanzas River at St. Augustine was the main entrance to the historic city, America's oldest port.

The preserved areas include salt marshes, mangrove tidal wetlands, oyster bars, estuarine lagoons, upland habitat, and marine environments.

The Matanzas River faces several pollution issues, mostly related to urbanization in St. Augustine and the northern portion of Anastasia Island.