Over the past 4 years, we have developed and extensively deployed the Calibrated, InfraRed, In situ Measurement System, or CIRIMS, for at-sea validation of satellite-derived sea surface temperature (SST). The project is funded by the NASA EOS Validation Program for validation of SST from MODIS, the MODerate resolution Imaging Spectroradiometer, aboard the EOS Terra and Aqua satellites. The design goals include autonomous operation at sea for up to 6 months and an accuracy of ±0.1°C. One of the most challenging aspects of the design is protection against the marine environment. We use commercially available infrared pyrometers and a precision blackbody housed in a temperature-controlled enclosure. The sensors are calibrated at regular interval using a cylindro-cone target immersed in temperature-controlled water bath, which allows the calibration points to follow the ocean surface temperature. An upward-looking pyrometer measures sky radiance in order to correct for the non-unity emissivity of water, which can introduce an error of up to 0.5°C. As part of our design strategy, we have evaluated the use of an infrared transparent window to completely protect the sensor and calibration blackbody from the marine environment. A total of three units have been fabricated and deployed at sea for over 700 days since 1998. We give an overview of the design and report on the performance of the CIRIMS in comparison to the Marine-Atmosphere Emitted Radiance Interferometer (M-AERI) which is the primary in situ validation instrument for MODIS.