A mesacanthid acanthodian, Promesacanthus eppleri n. gen., n. sp., is described based on specimens collected from the Lower Devonian (Lochkovian) Manon-the-Hill locality of the Mackenzie Mountains, Northwest Territories, Canada. The head and body resemble that of other mesacanthids, but unlike all other acanthodiforms, this new taxon has a small prepectoral spine anterior to the pectoral fin spine. This new mesacanthid also possesses ornamented, blade-like hyoidean gill covers, enlarged lobate head scales, fin spines with ribs and fine striations, a scapulocoracoid with a triangular coracoid portion and a dorsal blade which is elliptical in cross section, procoracoids that articulate with a rounded fossa on the anteromedial face of the scapulocoracoids, and jaws which articulate at a simple, single joint. Mesacanthids are thought to be basal among acanthodiforms and are grouped based on a phenetic argument and their shared retention of features which likely are primitive for acanthodiforms (most notably, enlarged head scales, blade-like hyoidean gill covers, and a single pair of prepelvic spines). Based on overall similarity, P. eppleri n. gen., n. sp. appears most similar to Mesacanthus mitchelli, but the relationships of P. eppleri n. gen., n. sp. within the Mesacanthidae have yet to be determined with a cladistic analysis.