The Cat Ba Biosphere Reserve is considered part of the Indochinese rainforest biogeographical province. It includes tropical humid forests, wetlands, mangroves and coral reef ecosystem types. The northern coastal region of Viet Nam has a humid, tropical monsoon climate with distinct wet and dry seasons. The area suffers from a high frequency of typhoons with 28% of all typhoons impacting the Vietnamese coastline appearing in this area.

There are 2,026 species found in the biosphere reserve including 199 phytoplankton, 89 zooplankton, 75 forms of seaweed, 23 types of mangrove, 160 corals, 475 zoobenthonic organisms, 119 fish, three seagrasses, 142 species of terrestrial fauna and 741 plants.

Many species are endemic and rare, the most significant being the globally important Golden-headed langur (Trachypithecus poliocephalus), restricted in range to Cat Ba Island, the largest of the 366 islands, and found nowhere else in the world.

The Hawksbill turtle (Eretmochelys imbricata) found in the Cat Ba region is inscribed on the IUCN Red List, while the seahorse (Hippocampus spp.) is also rare and threatened. Mangroves and seagrasses on Cat Ba function as nurseries for life in the bay and as protective buffers for local inhabitants from storm surges and typhoons driven by climate change and sea level rise.