Machairaspis, a genus of the Osteostraci, an extinct clade of jawless vertebrates, has been reported from localities in Spitsbergen, Norway, Podolia (Ukraine), and Prince of Wales Island (Nunavut, Canada). A new species of Machairaspis, described here from the Delorme Formation at the MOTH locality, extends the geographic range of this genus to the Mackenzie Mountains. Machairaspis serrata sp. nov. is known from a very small headshield and a fragment of an even smaller individual. The two specimens share distinctive, dorsally pointed serrations on the posterior margin of the exceptionally tall dorsal spine of the headshield. The new species can also be differentiated from other species of Machairaspis by characteristic rows of denticles along the margin of the headshield and is represented by much smaller individuals than are known for previously described species in the genus. Differences in the proportions of the cephalic spine illustrate allometric increase in relative spine height during growth, as previously documented for the related genus Superciliaspis. Machairaspis is here included with Scolenaspis in the subfamily Scolenaspidinae, sister to the subfamily Zenaspidinae in the family Zenaspididae. The presence of Machairaspis at MOTH is an additional similarity with Early Devonian fauna of Spitsbergen and is consistent with the Lochkovian (Early Devonian) age that has been proposed previously for the MOTH locality and with the suggestion that Spitsbergen represents part of the Laurentia terrane.