Abstract The thermal conductivity of natural seawater is measured at atmospheric pressure, temperature range of 10–90°C, and salinity range of 0–120g/kg. Samples of natural seawater is collected from the Arabian Gulf and prepared for different salinities by evaporation and dilution techniques. The experimental apparatus used in the measurements is a modified comparative radial-heat-flow concentric-cylinder apparatus that uses a steady state measurement technique. The method gives the ratio of the thermal conductivity of pure water to that of seawater. A simple correlation for the pure water thermal conductivity is developed based on the model provided by The International Association for the Properties of Water and Steam (IAPWS 2011) and has a maximum deviation of 0.05%. A simple model for the thermal conductivity ratio of pure water to seawater is developed and compared with the measured data, data in the literature, and a recent theoretical model for mixed-solvent electrolyte solutions. The maximum deviation between the present model prediction and the measurement is ±1% while the maximum deviation from the recent theoretical model is found to be 0.5%.