The seasonal changes of streamflow in the watershed reflect the dynamic characteristics of water resources within a year. In forested watersheds, it is generally accepted that forest cover change and climate variability are considered as two critical driving factors to affect hydrological processes. Thus, quantifying the effects of reforestation and climate variability on seasonal streamflow dynamics provides scientific basis for balancing regional carbon and water resources and developing sustainable forest management strategies in the context of global climate change. The Pingjiang watershed, which is located in the upper reach of Poyang Lake basin in subtropical China, was selected for this study. The entire study period consists of the reference period (1961-1985) and the reforestation period (1986-2006), based on the history of forest coverage dynamics. The method of Mann-Kendall trend analysis was used to detect whether trends in the hydroclimatic variables significantly varied over a long-term period. Meanwhile, the monthly aridity index (the ratio of potential evapotranspiration to effective precipitation) was applied to define two different seasons within a year as energy-limited season (January-June) and water-limited season (July-December), respectively. The extended Budyko model was used to quantify the relative contribution of reforestation and climate variability to the seasonal changes of streamflow in the studied watershed. First, based on Mann-Kendall method, streamflow in the water-limited season showed a significant increasing trend during the study period (1961-2006), but with no significant trend in hydrological and climatic variables for the energy-limited season. Second, reforestation decreased streamflow in the energy-limited season by 11.71 mm/a (24.40%), but increased that in the water-limited season by 12.27 mm/a (17.23%) compared with the reference period. By contrast, climate variability related streamflow was reduced by 36.28 mm/a (75.60%) in the energy-limited season but was increased by 58.94 mm/a (82.77%) in the water-limited season. These results demonstrated that effects of reforestation on streamflow can be cumulative over time and space. Reforestation had a positive role in regulating seasonal streamflow dynamics, while there could be a time-lag for the response of seasonal streamflow to reforestation. The effects of reforestation on streamflow was offset between the energy limited and water limited seasons, and the effects of climate variability on streamflow was similar with those of reforestation. Meanwhile, these results also demonstrated that the seasonal changes of streamflow in the Pingjiang watershed are primarily determined by climate change. However, reforestation may be another important driving force to affect seasonal streamflow variation.