The interference method is one of the most sensitive methods for measuring the refractive index of seawater. We developed a state-of-the-art density sensor for seawater measurements based on measuring the refractive index by the interference method. The resolution of the density sensor is 0.00006 kg/m3 for changing temperature at constant salinity and pressure, 0.00012 kg/m3 for changing salinity at constant temperature and pressure, and 0.00010 kg/m3 for changing pressure at constant temperature and salinity. These resolution values are the best in the history of seawater density measurements. The ultra high-resolution density sensor will contribute notably to climate research at full ocean depth and measurement of seawater sampled from the deep ocean, to research on metrology to establish the traceability of salinity measurements, and to submarine resource exploration to detect spatial changes in the absolute salinity anomaly by combining with conventional conductance-based salinity measurements.