Cat Ba is the largest of the 366 islands spanning 260km2 that comprise the Cat Ba Archipelago, which makes up the southeastern edge of Ha Long Bay in Northern Vietnam. Cat Ba island has a surface area of 285 km2 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.

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. Cat Ba Island is one of the only populated islands in Ha Long Bay, with roughly 13,000 inhabitants living in six different communes, and 4,000 more inhabitants living on floating fishing villages off the coast. The large majority of the population can be found in Cat Ba Town which is located at the southern tip of the Island (15 km south of the national park) and is the commercial center on the Island. Since 1997, Cat Ba Town has grown rapidly and has become a tourist hub for both the Island and greater Ha Long Bay.