Joffrichthys triangulpterus, sp. nov. is a new species of the family Osteoglossidae, and is described based on two nearly complete, and two partial, articulated specimens from the Sentinel Butte Formation in the Fort Union Group (Paleocene) of central North Dakota, USA. The familial placement is based on three diagnostic characters: uroneurals absent; six hypurals; and a kidney shaped opercle. The generic placement into Joffrichthys is based on two diagnostic characters and eight similar morphologies and meristics shared between the new species and previously described J. symmetropterus: two complete or partial neural spines present on preural 2; one spine or no spines present on ural 1; similar shapes of the maxillary, dentary, preopercle, opercle, and frontal; a similar body silhouette; and a similar vertebral and supraneural count. The specimens are identified as a new species by nine diagnostic characteristics distinguishing it from J. symmetropterus, the most closely related taxon: more posteriorly-placed pelvic fins; a greater dorsal fin ray count (30 total); a triangular dorsal and anal fin; asymmetrically positioned dorsal and anal fins (anterior pterygiophore insertions separated by 8 to 9 vertebrae); the third dorsal and fifth anal pterygiophores being the longest; and two complete spines on preural 2. The North Dakota fish is important because it represents a new species of Joffrichthys and represents the most southern occurrence of the genus to date.