The dependence of oxygen isotope fractionation on ice growth rate during the freezing of sea water is investigated based on laboratory experiments and field observations in McMurdo Sound, Antarctica. The laboratory experiments were performed in a tank filled with sea water, with sea ice grown under calm conditions at various room temperatures ranging from −5°C to −20°C. In McMurdo Sound, the ice growth rate was monitored using thermistor probes for first-year landfast ice that grew to ∼2 m in thickness. Combining these datasets allows, for the first time, examination of fractionation at a wide range of growth rates from 0.8 × 10−7 to 9.3 × 10−7 m s−1. In the analysis a stagnant boundary-layer model is parameterized using these two independent datasets. As a result, the optimum values of equilibrium pure-ice fractionation factor and boundary-layer thickness are estimated. It is suggested that a regime shift may occur at a growth rate of ∼2.0 × 10−7 m s−1. A case study on sea ice in the Sea of Okhotsk, where the growth rate is modeled by coupling the thermodynamic properties of the sea ice with meteorological data, demonstrates the utility of the fitted models.