The concentration-based (stoichiometric) equilibrium solubility product of ikaite (CaCO3·6H2O) in seawater and cryogenic seawater-derived brines was determined at 1 atm total pressure over the temperature range from −1.1 to −7.5 °C and the salinity range from 34 to 124 in temperature–salinity pairs representative of sea ice brines. The solubility measurements were obtained in solutions that were undersaturated and supersaturated with respect to ikaite by equilibration with CO2/N2 gas mixtures of known pCO2 (20–400 μatm). The solutions were then equilibrated with synthetic ikaite (seed) for up to 3 months in a closed system. Arrival of the solid–solution system at a long-term chemical equilibrium was indicated by attainment of constant chemical solution composition with respect to total dissolved calcium, total dissolved inorganic carbon, and total alkalinity. Using these measurements, the stoichiometric equilibrium solubility product of ikaite (View the MathML sourceKsp,ikaite∗=[Ca2+][CO32-], in View the MathML sourcemol2kgsolution-2) was determined, with the carbonate ion concentration computed from the measured total alkalinity and total dissolved inorganic carbon concentrations. The computed carbonate ion concentration and, by extension, the View the MathML sourceKsp,ikaite∗ are both contingent on solving the system of equations that describe the parameters of the CO2 system in seawater by extrapolation to the experimental salinity and temperature conditions. The results show that the View the MathML sourcepKsp,ikaite∗=-logKsp,ikaite∗ in seawater of salinity 34 at −1.1 °C was 5.362 ± 0.004 and that the View the MathML sourcepKsp,ikaite∗ in sea ice at the freezing point of brines of salinity greater than 34 can be described as a function of temperature (T , in K) by the equation, View the MathML sourcepKsp,ikaite∗=-15489.09608+623443.70216T-1+2355.14596lnT, in the temperature range of 265.15 K < T < 271.15 K (−8 °C < t < −2 °C). Brines of low pCO2 (20 μatm) yielded a much slower (>1 month) approach to chemical equilibrium when incubated without seeding ikaite crystals. Simple modeling indicated that ikaite should not precipitate from sea ice brines evolving under closed system conditions with respect to CO2 exchange. To facilitate ikaite precipitation, brine pCO2 reduction due to photosynthesis or CO2 degassing, or both, is necessary.