The genus Trematolobelia (Lobeliaceae) is endemic to the Hawaiian Islands. It resembles Lobelia, except for its peculiar dehiscence. On ripening the capsules shed their epidermis and soft mesocarp, exposing the outer bony endocarp which is pierced by numerous holes. On the exposed ridges or cliff faces, the slender stem is whipped back and forth by the wind. Centrifugal force ejects the seeds through the holes of the endocarp. This monograph presents nine species and three varieties of the genus, five on Hawaii, two on Oahu and Molokai, and one each on Kauai, Lanai, east Maui, and west Maui. Eight of them are new or are new placements.