The mangrove forest at Bako National Park occupies the tidal zones along the park's sheltered bays and river channels — a narrow but extraordinarily productive strip of vegetation where the forest meets the sea. Mangroves are among the most carbon-dense ecosystems on Earth and among the most threatened; Bako's protected mangroves represent one of the few intact examples remaining on the Sarawak coast. Three species dominate: Rhizophora apiculata, with its distinctive prop roots arching from the trunk into the mud; Bruguiera gymnorrhiza, with its buttress roots and cigar-shaped propagules; and the nipah palm, Nypa fruticans, which forms dense thickets in the most sheltered sections.
The mangrove mud flat exposed at low tide is alive with organisms invisible from above. Mud skippers (Periophthalmodon schlosseri) — fish that breathe air and move across mud using their pectoral fins as crutches — are abundant and entertaining to watch. Fiddler crabs (Uca spp.) emerge in hundreds at low tide, the males waving their single enlarged claw in species-specific courtship displays. Archer fish (Toxotes chatareus) lurk in the shallows, shooting jets of water at insects on overhanging mangrove leaves.
The mangrove fringe is central to the park's most reliable wildlife sighting: the evening assembly of proboscis monkeys. Troops of 20–30 animals descend from the forest interior to the mangrove and nipah at Telok Assam between 17:00 and 18:30 each day, drinking from the brackish water and feeding on the young nipah leaves before moving to sleeping trees. The mangrove boardwalk at park headquarters is built specifically to allow visitors to observe this behaviour without disturbing the animals or entering the mud.
The mangrove root system functions as a nursery for juvenile fish and crustaceans, as storm protection for the coastline, and as a food source for water monitors, white-bellied sea eagles, and the brahminy kite (Haliastur indus) — a smaller raptor common throughout the park's coastal zone. The mangrove boardwalk walk takes about 20 minutes and is flat and accessible to all fitness levels; it is the single most productive wildlife walk in the park relative to effort expended.