The Soucook River is a 29.2-mile-long (47.0 km) river located in central New Hampshire in the United States. It is a tributary of the Merrimack River, which flows to the Gulf of Maine. The Soucook River begins at the outlet of Rocky Pond on the border between the towns of Canterbury and Loudon, New Hampshire. The river flows south through gently rolling terrain, soon entering Loudon and passing near the New Hampshire Motor Speedway. The river passes through the village proper of Loudon, crossing a small dam, and continues south along a more suburban corridor on the outskirts of Concord. The river forms the boundary between Concord and Pembroke and ends at the Merrimack River downstream from Garvins Falls.