Killer whales (Orcinus orca) are distributed widely in all oceans, although they are most common in coastal waters of temperate and high-latitude regions. The species' distribution has not been fully described in the northwest Atlantic (NWA), where killer whales move into seasonally ice-free waters of the eastern Canadian Arctic (ECA) and occur year-round off the coast of Newfoundland and Labrador farther south. We measured stable oxygen and carbon isotope ratios in dentine phosphate (d18OP) and structural carbonate (d18OSC, d13CSC) of whole teeth and annual growth layers from killer whales that stranded in the ECA (n?=?11) and NWA (n?=?7). Source d18O of marine water (d18Omarine) at location of origin was estimated from dentine d18OP values, and then compared with predicted isoscape values to assign individual distributions. Dentine d18OP values were also assessed against those of other known-origin North Atlantic odontocetes for spatial reference. Most ECA and NWA killer whales had mean d18OP and estimated d18Omarine values consistent with 18O-depleted, high-latitude waters north of the Gulf Stream, above which a marked decrease in baseline d18O values occurs. Several individuals, however, had relatively high values that reflected origins in 18O-enriched, low-latitude waters below this boundary. Within-tooth d18OSC ranges on the order of 1-2‰ indicated interannual variation in distribution. Different distributions inferred from oxygen isotopes suggest there is not a single killer whale population distributed across the northwest Atlantic, and corroborate dietary and morphological differences of purported ecotypes in the region.