Big Oaks National Wildlife Refuge is a 50,000-acre (202 km2) wildlife refuge operated by the United States Fish and Wildlife Service in southeast Indiana, United States near Madison, Indiana. The refuge is the largest of Indiana's three National Wildlife Refuges (Muscatatuck National Wildlife Refuge near Seymour, IN and Patoka River National Wildlife Refuge and Management Area near Oakland City, IN), and is located in parts of Ripley, Jefferson, and Jennings counties. Over 200 species of birds and 46 species of mammals are found on the refuge.