The surface soil moisture controls surface energy budget. It is a key environmental variable in the coupled atmospheric and hydrological processes that are related to drought, heat waves and monsoon formation. Satellite remote sensing of soil moisture provides information that can contribute to understanding these processes and evaluating their impacts on regional and global climate. To identify soil moisture climate signals from seasonal to inter-annual time scales, soil moisture data sets have to be able to capture the global variability and be free from spatial and temporal biases. WindSat is the first spaceborne fully polarimetric microwave radiometer. The satellite was launched on January 6, 2003 into a sun-synchronous orbit with an altitude of 830 km. WindSat operates at 6.8, 10.7, 18.7, 23.8, and 37 GHz at a nominal incidence angle of 53 degrees and has swath width of about 1025 km. A multi-frequency algorithm has been implemented to simultaneously retrieve soil moisture, soil temperature and vegetation optical depth. In this investigation, soil moisture estimates were compared with long term in situ soil moisture observations. One set of data was from four soil moisture networks (Little Washita, OK; Little River, GA; Walnut Gulch, AZ; and Reynolds Creek, ID) that were developed by USDA/ARS and used as part of the Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) validation program. Each network is located in a different climatic region of the U.S. and provides estimates of the average soil moisture over highly instrumented experimental watersheds and surrounding areas that approximate the size of the sensor footprint. Soil moisture measurements have been made at these validation sites on a continuous basis since 2002, which provided an eight-year period of record for this analysis. In additional to the USDA/ARS soil moisture networks, we also used a single station long-term field campaign of the Surface Monitoring of the Soil Reservoir Experiment (SMOSREX) located near Mauzac, south of Toulouse, France. Land model simulations suggest that the point measurements of soil moisture at this location are representative of areal soil moisture in this Mauzac region. In order to examine potential synergism of WindSat and AMSR-E soil moisture data, we also compared Windsat measurements with AMSR-E soil moisture data derived using the NASA standard algorithm and the single-channel algorithm (SCA). The validation results and observed soil moisture climate signals will be presented at the meeting.