A new compact optical refractometer is presented to improve the measurement of the refractive index of seawater. These measurements are useful in oceanography to calculate density and salinity of oceans from empirical relations. This refractometer shows a lower temperature dependence and obtains a better absolute accuracy on salinity compared to the conductivity sensors which are used nowadays to assess seawater salinity. Tests and calibrations have been made in a temperature-stabilized seawater tank. They show that the prototype is capable of measuring seawater refractive index with a resolution of about ±4 × 10−7, equivalent to a salinity resolution of ±2 × 10−3 g kg−1.