The Blackwater River is a 3.1 mi tidal inlet in northeastern Massachusetts and southeastern New Hampshire in the United States. The river forms in a salt marsh in the northeastern corner of Salisbury, Massachusetts, by the convergence of the Little River and Dead Creek. Heading north, the river quickly enters Seabrook, New Hampshire and continues to flow through salt marsh until it reaches Hampton Harbor, northwest of Seabrook Beach, where it joins the Hampton River.