The Okhotsk people were sedentary hunter-gatherer-fishers who lived and prospered in Sakhalin, Hokkaido, and the Kurile Islands during the fifth–thirteenth centuries AD. They expanded rapidly along the north-eastern coast of Hokkaido where archaeological evidence suggests an increasing dependence on hunting marine mammals. In this study, we present the results of carbon and nitrogen isotope analyses of 18 faunal (including two domesticated dogs) and 58 adult human skeletons excavated from the Moyoro site of the Okhotsk culture in eastern Hokkaido. Although the mean human isotope ratios did not differ between sexes, the variances of carbon isotope ratios were significantly greater in males. Carbon and nitrogen isotope ratios suggest that the Moyoro humans and dogs heavily depended on marine foods for their dietary protein intake. The Bayesian mixing model suggests that humans obtained a maximum of 80–90% of their dietary protein from marine mammals, whereas domesticated dogs obtained 2–33%, 3–40%, and 5–45% of dietary protein from brackish-water fish, marine fish, and marine mammals, respectively. This suggests an avoidance of significant dietary overlap between the sympatrically living humans and dogs at the Moyoro site. Significant maritime adaptation would have enabled the subsistence of the Okhotsk people in the harsh northern environment of Hokkaido.