The plant bug genus Orthotylus has undergone extensive insular radiation in the Hawaiian Islands. Based on male paramere structure and dorsal setiferation, at least four major clades appear to be present, each of which has sequentially tracked particular host plant families along the island chain in geological and evolutionary time. In this work, 21 new species of Hawaiian Orthotylus are described, arranged by host plant affiliation. The new species and their host plant associations are as follows: O. pisoniae from Oahu, on Pisonia umbellifera and Pisonia sandwicensis; O. pisonioides, from Hawaii, on Pisonia brunoniana; O. hedyoti from Oahu, on Hedyotis terminalis; O. hedyotiopsis from Oahu, on Hedyotis acuminata; O. hedyoticola from Hawaii, on Hedyotis terminalis; O. psychotriae from Hawaii, on Psychotria hawaiiensis; O. psychotrioides from Hawaii, on Psychotria hawaiiensis; O. psychotricola from Oahu, on Psychotria mariniana; O. coprosmae from Maui, on Coprosma foliosa and Coprosma pubens; O. coprosmoides from Oahu, on Coprosma ochracea; O. ilicis from Maui, on Ilex anomala; O. neoilicis from Hawaii, on Ilex anomala; O. urerae from Oahu, on Urera kaalae; O. pipturi from Hawaii, on Pipturus albidus; O. pipturoides from Maui, on Pipturus albidus; O. pseudotantali from Maui, on Pipturus albidus; O. clermontiae from Hawaii, on Clermontia clermontioides; O. xylosmae from Hawaii, on Xylosma hawaiiense; O. diospyri from Hawaii, on Diospyros sandwicensis; and O. hibisci from Oahu, on Hibiscus arnottianus. Redescriptions are also given for 5 previously described species: O. kanakanus (Kirkaldy) from Hawaii, on Coprosma rhynchocarpa; O. perkinsi Kirkaldy from Hawaii, on Ilex anomala; O. tantali (Perkins) from Oahu on Pipturus albidus; O. iolani Kirkaldy from Hawaii, on Clermontia clermontioides; and O. daphne (Kirkaldy) from Oahu on Xylosma hawaiiense. Shaded dorsal habitus drawings are provided for O. hedyotiopsis and O. psychotrioides. Figures of the male genitalic structures and dis