Bahía San Sebastián is a vast, shallow, and protected bay on the eastern Atlantic coast of Tierra del Fuego, Argentina, situated north of the city of Río Grande. It is characterized by its extensive intertidal zones, featuring low coasts with saline, muddy, and sandy soils. The bay is partially enclosed by the El Páramo peninsula, creating a sheltered marine environment. It is renowned for its extreme tidal range, with one of the largest tidal amplitudes in the region, where the 36-foot tide can leave marine life stranded across wide flats. The area forms part of the Costa Atlántica Tierra del Fuego Provincial Nature Reserve, highlighting its significant ecological value for shorebirds and marine life. The surrounding landscape is a mix of tidal plains, rocky reefs, and glacial erratic blocks near Punta Sinaí.