The East Branch Croton River is a tributary of the Croton River in Dutchess, Putnam, and Westchester counties in the state of New York. It lies within the Croton River watershed and is part of the New York City water supply system's Croton Watershed.
The river rises in the town of Pawling in Dutchess County, flowing west and then south through the village of Pawling. It crosses into Putnam County, flowing through the Great Swamp in the town of Patterson. It then flows south, west and southwest through the town of Southeast, passing through the East Branch Reservoir, the village of Brewster, and the Diverting Reservoir. Finally, it crosses into Westchester County, joining the West Branch Croton River at the hamlet of Croton Falls in North Salem to form the main Croton River.