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Abstract Here, we report an optical sensor for measuring millimeter-scale thermohaline density variations in the ocean. The instrument is based on a fiber Fabry–Perot white light interferometer, which can resolve the refractive index of water to better than 2 × 10⁻⁸within a sample volume smaller than 1 mm³at a sample rate of 500 Hz. This equates to detectable Absolute Salinity variations of 0.0001 g kg⁻¹, temperature variations smaller than 0.0007°C, and density variations of 0.000 07 kg m⁻³. Data collected from laboratory characterization and a field deployment suggest the sensor could be useful as a gradiometer for measuring diapycnal mixing down to submillimeter scales. While obtaining high absolute accuracy in refractive index and density was not a primary consideration, the sensor was designed with such an eventual goal in mind, and various aspects of achieving it are discussed. Significance StatementAccurately measuring and modeling ocean salinity at the smallest scales remains an unsolved problem in oceanography. Such measurements would be beneficial to fully understand the vertical transport of heat and salt in the ocean, the effects of melting Arctic ice, and overall oceanic mixing. By achieving very high-resolution refractive index measurements at millimeter spatial scales, the in situ sensor technology reported here could begin to address these challenges.