Lyon Arboretum, Botanical garden in Manoa Valley, Hawaii, US.

Lyon Arboretum is a botanical garden with 193 acres of tropical plants and Hawaiian native species nestled in the rainforest climate of Oahu. The site includes garden paths, research facilities, and collections of rare island flora throughout its lush mountain setting.

The Hawaiian Sugar Planters' Association founded the Manoa Arboretum in 1918 to restore watershed retention and test tree species for reforestation. Its first director, botanist Harold L. Lyon, planted nearly 2,000 tree species. In 1953 the association conveyed the site to the University of Hawaiʻi for $1,000, on condition it remain an arboretum; it was renamed for Lyon after his death in 1957.

The ethnobotanical, native ecosystems and Hawaiian sections display native and Polynesian cultivated and wild species, including Pritchardia palms. The collection grew from plants Dr. Harold L. Lyon assembled while testing species for reforestation, and today the Hawaiian Rare Plant Program's seed laboratory banks seeds of threatened taxa.

Access is by reservation only, Monday through Friday from 9 am to 3 pm; the garden is closed on weekends. The entrance sits at the top of Mānoa Road in Mānoa Valley, and the grounds climb from about 137 to 564 metres, so expect steep, often muddy trails. Book ahead before travelling up the valley.

The site houses over 5,000 plant species within its mountain microclimate, some so rare that they are grown only here in a specialized propagation facility. This ability to reproduce endangered plants makes the location a critical refuge for Hawaiian flora.