The most recent source of satellite-based global soil moisture is the European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission. A thorough validation must be conducted to insure product quality that will in turn support the widespread utilization of the data. This is especially important since SMOS utilizes a new sensor technology and is the first passive L-band system in routine operation. We conducted a validation of the SMOS soil moisture product using a set of four in situ soil moisture networks located in the U.S. These ground-based observations are combined with retrievals based upon another satellite sensor, the Advanced Microwave Scanning Radiometer (AMSR-E). The watershed sites are highly reliable and address scaling with replicate sampling. Results of the validation analysis indicate that the SMOS soil moisture estimates are approaching the level of performance anticipated, based on comparisons with the in situ data and AMSR-E retrievals. The ascending pass overall root mean square error of the SMOS soil moisture estimates is 0.043 m3/m3 for the watershed networks. There are bias issues at some sites that need to be addressed as well as some outlier responses. Analyses indicated that active or recent rainfall can contribute to interpretation problems when assessing algorithm performance, which is related to the contributing depth of the satellite sensor. An investigation of the vegetation optical depth retrievals provided by the SMOS algorithm indicated that, for the watershed sites, these are not a reliable source of information on the vegetation canopy. The SMOS algorithms will continue to be refined as feedback from validation is evaluated and it is expected that the SMOS estimates will improve.