The Mecyclothorax carabid beetle fauna of Haleakalā volcano, Maui Island, Hawai'i is taxonomically revised, with 116 species precinctive to Haleakalā recognized, 74 newly described. Species are classified into 14 species groups, with the newly described species arrayed as follows: 1, M. constrictus group with M. perseveratus sp. n.; 2, M. obscuricornis group with M. notobscuricornis sp. n., M. mordax sp. n., M. mordicus sp. n., M. manducus sp. n., M. ambulatus sp. n., M. montanus sp. n., M. waikamoi sp. n., M. poouli sp. n., and M. ahulili sp. n.; 3, M. robustus group with M. affinis sp. n., M. anchisteus sp. n., M. consanguineus sp. n., M. antaeus sp. n., M. cymindulus sp. n., and M. haydeni sp. n.; 4, M. interruptus group with M. bradycelloides sp. n., M. anthracinus sp. n., M. arthuri sp. n., M. medeirosi sp. n., M. inconscriptus sp. n., and M. foveolatus sp. n.; 5, M. sobrinus group with M. foveopunctatus sp. n.; 6, M. ovipennis group with M. subtilis Britton & Liebherr, sp. n., M. patulus sp. n., M. patagiatus sp. n., M. strigosus sp. n., M. takumiae sp. n., M. parapicalis sp. n., M. mauiae sp. n., M. subternus sp. n., M. flaviventris sp. n., M. cordaticollaris sp. n., and M. krushelnyckyi sp. n.; 7, M. argutor group with M. ommatoplax sp. n., M. semistriatus sp. n., M. refulgens sp. n., M. argutulus sp. n., M. planipennis sp. n., M. planatus sp. n., and M. argutuloides sp. n.; 8, M. microps group with M. major sp. n., M. xestos sp. n., M. orbiculus sp. n., and M. contractus sp. n.; 9, M. scaritoides group with M. scarites sp. n., M. timberlakei sp. n., M. crassuloides sp. n., M. crassulus sp. n., M. gracilicollis sp. n., and M. dispar sp. n.; 10, M. haleakalae group with M. reiteratus sp. n., M. splendidus sp. n., M. bacrionis sp. n., and M. simpulum sp. n.; 11, M. vitreus group with M. kipwilli sp. n., M. kipahulu sp. n., M. kaumakani sp. n., and M. kuiki sp. n.; 12, M. montivagus group with M. rex sp. n.; 13, M. ducalis group with M. aquilus sp. n., M. invisitatus sp. n., M. longidux sp. n., and M. brevidux sp. n.; and 14, M. palustris group with M. hephaestoides sp. n., M. oculellus sp. n., M. bicoloris sp. n., M. bicoloratus sp. n., M. bilobatus sp. n., M. palustroides sp. n., M. filipoides sp. n., M. nanunctus sp. n., M. tauberorum sp. n., and M. pau sp. n. Mecyclothorax integer Sharp, stat. n. is recognized as a species distinct from M. interruptus Sharp. Because type series for species described by Blackburn, Karsch, and Sharp are most often divided among geographically remote collections, lectotypes are designated to stabilize the nomenclature. The radiation includes numerous cryptic sibling species best diagnosed using male genitalia, and photographs are used to represent the male genitalic variability observed among numerous dissected individuals. The large number of new species is based on substantial new collections made from all quarters of the mountain. The dense geographic sampling allows fine-scale discrimination of species boundaries, elucidating the geographic disjunctions that are associated with speciation within this hyperdiverse radiation. Disjunctions between closely related species precinctive to various areas of the mountain are not congruent across the different lineages of the radiation, indicating differential responses by the various lineages to past geological and geographical events. Of the 62 1' latitude × 1' longitude grid cells on Haleakalā that are occupied by Mecyclothorax beetles, 22 house 10 or more species, and 9 house 20 or more species. This substantial level of sympatry, associated with occupation of diverse microhabitats by these beetles, provides ample information useful for monitoring biodiversity of the natural areas of Haleakalā.