We analyzed streamflow records from more than 300 stations across Thailand, a tropical country in Southeast Asia. Temporal changes in runoff yield were assessed over the 1960-2015 period, highlighting a prominent downward trend over the last two decades. To identify potential drivers of these changes, gridded data products representing precipitation, temperature, evapotranspiration, and land cover were also assessed. We found that runoff yield is primarily driven by annual precipitation, which has experienced an unprecedented decline since 2010. Two sub-regions with particularly robust data coverage reflected a spatial contrast in hydrologic response: a more consistent response of runoff yield to precipitation is observed in the sub-region characterized by a high density of forest cover relative to the region characterized by high cropland cover. This feature underscores the need to take land use and irrigation practices into account when forecasting, and determining management strategies for, tropical river streamflow in a warming climate.