Climate change, as one of the most challenging issues of the twenty-first century, poses a serious threat to the sustainability of water resources. The present study was conducted to investigate the effects of climate change on temperature regimes, precipitation patterns and the occurrence of hydro-climatic extreme events within the Tajan River basin, northern Iran. For this purpose the SDSM downscaling model was employed under the Sixth Assessment Report (AR6) and the SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios for two future periods (2025–2055 and 2065–2095) at the Soleyman-Tangeh and Kiassar stations. Design precipitation values were also estimated using statistical analysis and the EasyFit distribution-fitting model. The results indicate that monthly minimum and maximum temperatures increase in all scenarios, with stronger warming under high-emission scenarios. Moreover, the precipitation regime shifts in the future, with increases in winter and autumn precipitation and a noticeable decrease in spring precipitation. During the 2025–2055 period, the intensity of extreme precipitation events rises, increasing the likelihood of destructive floods; however, in the 2065–2095 period, a substantial decline in design precipitation is observed across all return periods, which may reduce the potential for intense rainfall events but increase the risk of widespread droughts. The decline in design precipitation during the late-century period reflects a shift in climate conditions toward fewer extreme rainfall events and generally drier conditions. These findings highlight intensified climatic variability, greater concentration of precipitation in short-duration extreme events, and increased surface runoff, all of which impose additional pressure on water-resources management.