The Global Satellite Mapping of Precipitation (GSMaP) project provides semi-global and hourly rain rate estimates based on multi-satellite measurements. However, the quality of GSMaP estimates degrades when no microwave radiometers are available or when a cloud moving vector is applied. This study proposes a new rain detection method for the interval between microwave measurements based on variations in surface soil moisture. The new method is tested with Tropical Rainfall Measuring Mission Microwave Imager data and evaluated with rain gauge measurements from the Oklahoma Mesonet. Generally, 10 GHz with horizontal polarization (10H) is the optimal channel to detect temporal increases in surface soil moisture induced by rainfall. Compared with GSMaP, the performance of hourly rain detection in the new method is not high. However, the new method is expected to perform better if measurement intervals are shortened by use of multiple microwave radiometers. The new method is good for less vegetated areas where GSMaP performance is reduced. Currently, the new method is mostly independent of GSMaP as 10H is not used for rain retrieval over land. Incorporation of the new method into GSMaP is expected to improve the latter.