The Devonian marine strata of southwestern Ontario, Canada, have been well documented geologically, but their vertebrate fossils are poorly studied. Here we report a new onychodontiform (Osteichthyes, Sarcopterygii) Onychodus eriensis n. sp. from the Dundee Formation (Eifelian–Givetian boundary, 390–387 Ma) of southwestern Ontario represented by two well-preserved onychodontiform lower jaws. The most complete specimen consists of a large (28 cm), well-preserved right jaw with most of the dentition present. The dentary has 50 teeth, not including the parasymphysial tusk whorl, which is poorly preserved but consists of at least three tusks. The anteriormost teeth of the dentary are also not complete, but the second dentary tooth is notably procurved. The posterior teeth are conical and approximately equal in size for much of the length of the tooth row. Onychodus eriensis n. sp. differs from the closely related contemporary species Onychodus sigmoides and all other onychodonts in that it has a strong dorsal curvature of the anterior dentary ramus and marked anterior expansion of the dentary. An expanded phylogenetic analysis of Devonian onychodontiforms suggests that O. eriensis is closely related to Onychodus jandamarrai. The new material indicates that Onychodontiformes is more diverse than previously recognized, and that further analysis of vertebrate remains from southwestern Ontario will lead to additional insights into the diversity of Devonian sarcopterygians.