We present a molecular phylogenetic and taxonomic revision of the genus Fissurina (Graphidales) in Hawaiʻi. Compared to eight previously reported species, we distinguish a total of 26 species. Only four of the previously reported species are tentatively confirmed, namely F. columbina, F. globulifica, F. stromatoides, and F. zahlbruckneriana. An additional four species are new records for the archipelago, including F. consentanea, F. crassilabra, F. deficiens, and F. hyalinella. The following 18 species are described as new to science: F. alani sp. nov., differing from F. chrysocarpoides in the larger ascospores; F. biliana sp. nov., differing from F. hyalinella in the longer ascospores; F. brevicarpa sp. nov., differing from F. hyalinella in the short, broad, somewhat gall-forming lirellae; F. fissa sp. nov., differing from F. tachygrapha in the thicker thallus, the weak apical carbonization of the labia, and the relatively narrower ascospores; F. fuscella sp. nov., differing from F. tectigera in the apically carbonized labia and the shorter ascospores becoming brown when mature; F. fuscispora sp. nov., differing from F. dumastii and similar species in the ascospores becoming brown, and from F. fujisanensis in the non-carbonized labia; F. graphidoides sp. nov., differing from F. gomphospora in the dark labia contrasting with the lighter thallus, and from F. decipiens in the more distinct, dark grey labia lacking a black line along the slit; F. implicatoides sp. nov., differing from F. capsulata in the thin, largely endoperidermal thallus and in the shorter, broader ascospores; F. kokeensis sp. nov., differing from F. hyalinella in the presence of distinct (albeit thin) labia and in the weakly amyloid ascospores; F. mauiensis sp. nov., differing from F. fissa in the slightly smaller ascospores; F. montiformis sp. nov., differing from F. alligatorensis in the minutely gall-forming lirellae with concealed disc and the somewhat narrower ascospores; F. nigriannulata sp. nov., differing from F. hyalinella in the irregular black rim bordering the gaping disc of the lirellae; F. papillosa sp. nov., differing from F. insidiosa in the even thallus with numerous, tiny papillae; F. sandwicensis sp. nov., differing from F. fissa in the larger, muriform ascospores; F. serpentina sp. nov., differing from F. insidiosa in the very long, radiating lirellae; F. subcoarctata sp. nov., differing from F. coarctata in the more robust, less crowded lirellae; F. subcrassilabra sp. nov., differing from F. crassilabra in the less emergent lirellae with concealed disc and in the only weakly amyloid ascospore; and F. subtimidula sp. nov., differing from F. fissa in the much larger ascospores and from F. sandwicensis in the 3-septate ascospores. Based on our study, the number of species of Fissurina now known from Hawaiʻi has more than tripled and potential endemism has increased from zero to 78%. A key to all species now reported for Hawaiʻi is presented. This study is dedicated to our esteemed colleague and friend, André Aptroot, on the occasion of his 65th birthday, for his numerous contributions to the taxonomy of tropical crustose lichens. Key words: Endoperidermal, Fissurinaceae, Fissurinoideae, Graphidaceae.