Shearwater Marsh is a wetland area that covers about 150 acres. The marsh is part of the South River watershed and provides important ecological functions, such as water quality improvement, flood control, and habitat protection. The marsh is also a popular destination for outdoor recreation, including hiking, kayaking, birdwatching, and photography. Shearwater Marsh is managed by the Massachusetts Department of Conservation and Recreation and is open to the public year-round.

Shearwater Marsh is located on the South River north of MA-139.