Cat Ba Island is the largest of the 367 islands spanning 260 km2 (100 sq mi) that comprise the Cat Ba Archipelago, which makes up the southeastern edge of Lan Ha Bay in Northern Vietnam. Cat Ba island has a surface area of 285 km2 (110 sq mi) and maintains the dramatic and rugged features of Ha Long Bay. It is commonly used as an overnight hotel stop on tours to Ha Long Bay run by travel agents from Hanoi. This island belongs to Haiphong city.
Cat Ba is the largest island in the Bay and approximately half of its area is covered by a National Park, which is home to the highly endangered Cat Ba langur. The island has a wide variety of natural ecosystems, both marine and terrestrial, leading to incredibly high rates of biodiversity. Types of natural habitats found on Cat Ba Archipelago include limestone karsts, tropical limestone forests, coral reefs, mangrove and sea grass beds, lagoons, beaches, caves and willow swamp forests.