Abstract Elevated atmospheric CO2 enhances vegetation productivity through direct physiological and indirect climate‐mediated pathways. However, how the balance between these mechanisms evolves under continued CO2 rise remains unclear. Here, we use CMIP6 Earth system models to investigate future changes in gross primary productivity (GPP) over Northeast Asia (NEA). Under SSP5–8.5, GPP is projected to increase throughout the twenty‐first century. However, its sensitivity to rising CO2 weakens markedly from early to late century. This weakening is associated with declines in both direct physiological fertilization and indirect climate effects, with the largest reductions in the semi‐arid to humid transition zone. Notably, this zone exhibits the most pronounced shift from direct to climate‐mediated regulation of productivity, associated with dryness. Continued dryland expansion may exacerbate ecosystem vulnerability. Overall, our results suggest that although GPP in NEA is likely to continue increasing under rising CO2, its future trajectory will be increasingly constrained by hydroclimatic stress.