We present a validation study of sea surface temperature (SST) retrievals from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). TMI SSTs are calculated using a radiative transfer-based retrieval algorithm, which precisely accounts for SST and wind effects on surface emissivity as well as atmospheric effects on brightness temperatures. TMI SSTs are compared to 4 years of the National Data Buoy Center, Tropical Atmosphere Ocean/Triangle Trans-Ocean Buoy Network (TAO/TRITON), and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) moored buoy in situ SSTs. The microwave SSTs are shown to have a mean bias of -0.07¿C and a standard deviation of 0.57¿C when compared to the TAO/TRITON and PIRATA SSTs. A discussion of time series and dependencies of the accuracy of the TMI SSTs in the presence of varying wind, cloud, and water vapor for each of the buoy arrays is presented. TRMM's precessing equatorial orbit allows the diurnal variability to be determined, revealing midafternoon warming of the surface layer at wind speeds less than 6 m s-1. Low-wind TAO/TRITON collocations show diurnal warming in both the TMI and buoy retrievals, with TMI's diurnal peak larger and 1 hour before the buoy peak. This decoupling of the skin-bulk SSTs results in larger standard deviations for daytime low-wind situations. This result has direct implications for future in situ validation studies, in situ-based regression algorithms, and future blended infrared/microwave/in situ SST products.