The Medieval Warm Period (MWP; 800-1300 AD) represents a recent period of warm climate that can be compared to today's warming trend. However, the spatial and temporal variability inherent in the MWP makes it difficult to differentiate between global climate trends and regional variability. Acquiring high-resolution temperature data from this period will allow for increased understanding of temperature variability during this climate interval. We used oxygen isotope ratios preserved in archaeological limpet shells (Patella vulgata) collected from Viking aged midden deposits as a proxy for sea surface temperature. Samples were micromilled to achieve submonthly resolution. Summer and winter temperatures averaged 13.4 ± 0.7 °C and 5.9 ± 0.8 °C, respectively. When compared to regional data from NOAA (12.40 ± 0.39 °C and 7.76 ± 0.44 °C) from 1961-1990, MWP summer temperatures were warmer than current averages, and winter temperatures were cooler. Our results indicate that MWP seasonality was greater than that observed today.