Waimanu Valley, Valley in the Hāmākua district, Hawaiʻi island, United States

Waimanu Valley is a remote valley on the northeast coast of Hawaiʻi island, in the Hāmākua district. Waimanu Stream drains a watershed running from Kohala Mountain to the Pacific at Waimanu Bay, joined by tributaries such as Waihīlau Stream. Waihīlau Falls drops from about 3,000 feet (910 m) to about 400 feet (120 m) elevation, one of the tallest single drops in the United States; Waiʻilikahi Falls and Lahomene Falls also lie in the valley.

Ancient Hawaiians settled in this valley and developed land management practices using a traditional system called ahupua'a to organize resources. The small settlement shaped the region's history for centuries before the area became increasingly remote and isolated.

Waimanu means "bird water" or "river of birds" in Hawaiian. In the time of Ancient Hawaii the valley was an ahupuaʻa, an ancient land division that held a small village. The name Waihīlau for the tributary stream and its falls comes from wai hī lau, "many trickling waters", because in heavy rain the stream splits into many smaller streams that overflow the cliff.

Access to the valley requires hiking the Muliwai Trail, a challenging journey with steep climbs and multiple stream crossings along the way. Bring plenty of water and supplies since there are no facilities on the route and weather can change quickly in the mountains.

Waimanu Valley's dike system of hard lava rock forces groundwater dropped by the tradewinds into the valley, which is why it differs from the shallower valleys directly to the west. The ridge above is administered as the Puʻu O ʻUmi Natural State Area Reserve.