Cat Ba National Park is located in Cat Hai district, Hai Phong city. The national park is centred on Cat Ba island, a 28,500 ha island, which lies 20 km due east of Hai Phong city and immediately to the west of Halong bay. The national park also incorporates some of the small islands and marine waters situated to the east of Cat Ba island.
Like Halong bay, the landscape of Cat Ba National Park is dominated by karst limestone islands rising abruptly from the sea. The topography is rugged and marked by steep outcrops and areas of bare rock. The national park ranges in elevation from sea level to 331 m at the summit of Mount Cao Vong.
As is typical in well developed karst landscapes, drainage patterns are complicated by subterranean passages, which probably account for most of the drainage in the national park. The centre of Cat Ba island is no more than 5 km from the coast, and surface drainage is poorly developed and seasonal.