Forest hydrological and biogeochemical processes are highly dependent on soil water. At the Marcell Experimental Forest, seasonal patterns of soil moisture have been monitored at three forested locations since 1966. This unique, long-term data set was used to analyze seasonal trends in soil moisture as well as the influence of time-lagged precipitation and modified Thornthwaite modeled potential evapotranspiration (PET) on seasonal soil moisture at three depths (0-15, 76-107, and 198-229 cm). Despite no change in precipitation during the 45-yr record, mean annual soil moisture from 0 to 228.6 cm has declined (p < 0.05). Precipitation minus PET was found to account for >50% of the variability in seasonal soil moisture (p < 0.001). Our findings suggest that further increases in mean annual temperature and evapotranspiration may lead to decreases in soil moisture. These decreases could negatively impact forested ecosystems in northern Minnesota.