In a combined experimental and model study, we investigated effects of surface topography (relief) on the thermal L-band emission of a sandy soil. To this end, brightness temperatures of two adjacent footprint areas were measured quasi-simultaneously with an L-band radiometer at the observation angle of 55° relative to nadir for one year. One footprint featured a distinct relief in the form of erosion gullies with steep slopes, whereas the surface of the second footprint was smooth. Additionally, hydrometeorological variables, in situ soil moisture and temperature were measured, and digital terrain models of the two scenes were derived from terrestrial laser scanning. A facet model, taking into account the topography of the footprint surfaces as well as the antenna’s directivity, was developed and brightness temperatures of both footprints were simulated based on the hydrometeorological and in situ soil data. We found that brightness temperatures of the footprint with the distinct surface relief were increased at horizontal and decreased at vertical polarization with respect to those of the plane footprint. The simulations showed that this is mainly due to modifications of local (facet) observation angles and due to polarization mixing caused by the pronounced relief. Measurements furthermore revealed that brightness temperatures of both areas respond differently to changing ambient conditions indicating differences in their hydrological properties.