Bahía de San Antonio is a large, protected bay on the northern Patagonian coast within the Golfo San Matías in Río Negro Province, Argentina. This expansive coastal ecosystem features a diverse mosaic of habitats including sandy beaches, shell beaches, extensive sandbanks, and a vast intertidal marsh system. At low tide, the bay exposes a complex network of espartillar-crab beds and large sandy plains crisscrossed by tidal channels that drain into the sea. The surrounding terrestrial landscape is a shrubby steppe. The area is a designated Protected Natural Area, encompassing populated centers like Puerto de San Antonio Este, San Antonio Oeste, and the tourist town of Las Grutas. It is a region of significant ecological importance, supporting a rich array of marine life, birds, and mammals, while also sustaining a historical artisanal fishing tradition.