Coprosma kawaikiniensis K.R. Wood, Lorence & Kiehn (Rubiaceae), a rare endemic tree from Kaua'i, Hawaiian Islands, is described and illustrated along with a previously undescribed endemic plant community, the Dubautia-Sadleria shrubland-fernland (DSSF). The new species differs from Hawai'i congeners by its combination of opposite, long, elliptic to narrowly elliptic or ovate-elliptic leaves with revolute margins; caducous stipules 7-10 mm long, externally glabrous, densely hirtellous-pilose near the margins of the inner surface; unbranched inflorescences with peduncles 20-28 mm long; flowers 6-8 per cluster; and persistent calyx tube with 4-8 irregular dentate lobes. Known only from the windward slopes and ridges of southeastern Kaua'i below the Kawaikini summit, Coprosma kawaikiniensis falls into the IUCN Critically Endangered (CR) Red List category.
Keywords: Coprosma; Critically Endangered; Dubautia-Sadleria shrubland-fernland; Hawaiian Islands; Kaua‘i; Rubiaceae; high endemism; new species.