Measurements of soil moisture at various spatial and temporal scales are needed to study the water and carbon cycles. While satellite missions have been planned to measure soil moisture at global scales, these missions also need ground-based soil moisture data to validate their observations and retrieval algorithms. Here, we demonstrate that signals routinely recorded by Global Positioning System (GPS) receivers installed to measure crustal deformation for geophysical studies could be used to provide a global network of soil moisture sensors. The sensitivity to soil moisture is seen in reflected GPS signals, which are quantified by using the GPS signal to noise ratio data. We show that these data are sensitive to soil moisture variations for areas of 1000 m2 horizontally and 1-6 cm vertically. It is demonstrated that GPS signals penetrate deeper when the soil is dry than when it is wet. This change in penetration or ¿¿reflector¿¿ depth, along with the change in dielectric constant, causes the GPS signal strength to change its frequency and amplitude. Comparisons with conventional water content reflectometer sensors show good agreement (r2=0.9 to 0.76) with the variation in frequencies of the reflected GPS signals over a period of 7 months, with most of the disagreement occurring when soil moisture content is less than 0.1 cm3/cm3.