This study investigated the environmental determinants of NO₃⁻ concentrations from undisturbed mountain streams in Kyushu Main Island, Japan.  869 water samples were collected between March 2016 and May 2023 under the baseflow condition. Random Forest (RF) regression model was used to identify the important environmental factors affecting stream NO3- concentrations. The RF result revealed that annual precipitation is the most significant determinant, negatively correlated with NO3- concentrations. This suggests that higher precipitation enhances NO3- flushing from forest soils, reducing NO3- availability during subsequent baseflow periods. Atmospheric N depositions derived from agricultural emissions and fuel combustion are also significantly influenced on stream NO3-. Stream NO₃⁻ concentrations tended to increase when total annual N deposition exceeded approximately 12 kg N ha⁻¹ year⁻¹. These findings highlight the importance of climatic variables, particularly precipitation, and N depositions in determining stream NO3- concentrations in the wider area, providing a valuable framework for predicting and mitigating N pollution in similar ecosystems globally.