Bass River is an estuary and village in South Yarmouth, Massachusetts. The estuary meets Nantucket Sound at its broadest point. Its brackish, northwestern end flows into Follins Pond, from there going further north as a brook, to Mill Pond then an underground stream that approximately bisects Cape Cod.
The beaches are popular for kite boarding and windsurfing. There is a large sea grass covered land mass in the middle of the river. This is locally known as Marsh Island.