Melicopeiolensis (Rutaceae), a new tree species from Kaua'i, Hawaiian Islands.

A newly-discovered endemic tree species of Melicope from Kaua'i, Hawaiian Islands, is described and illustrated with notes on its distribution, ecology, conservation status and phylogenetic placement. A modification to the existing key to Hawaiian Melicope is also provided. Melicopeiolensis sp. nov. is a member of Stone's Megacarpa group having carpels connate at base, capsules 4-lobed and leaves usually opposite. The new species differs from its Hawaiian congeners by its unique combination of abaxially glabrate to pilose-pubescent leaves with petioles up to 70 mm long, ramiflorous and axillary inflorescences, sepals on staminate flowers 0.3-0.5 mm long, capsules with green and purple streaking, 10-14 mm wide and seeds 3-3.5 mm long. Since its discovery in 2021, 15 individuals have been documented within a single remote windward hanging valley below the Kawaikini Summit of Kaua'i. Melicopeiolensis represents a new Critically Endangered (CR) single island endemic species in need of conservation.