The stomatal conductance of terrestrial vegetation affects the exchange fluxes of water and carbon at the land surface. Here, a dynamic vegetation model coupled to a climate model of intermediate complexity is used to demonstrate that an optimum stomatal conductance exists at which the rate of photosynthesis, and therefore vegetation productivity, is at a maximum. This maximum originates from two competing drivers that affect the rate of photosynthesis: (i) increased supply of atmospheric carbon dioxide with increased stomatal conductance and (ii) increased cloud cover, which reduces the supply of sunlight. The simulated climate with optimized stomatal conductance is close to the model's control simulation, but vegetation productivity is substantially enhanced. The existence of this optimum has potentially important implications for the adaptation of terrestrial productivity to different climates. If vegetation maintains this optimum not only in the present-day, but also adapts to it during past (and future) climates, not considering this optimum and how it changes leads to a general underestimation of terrestrial productivity under different climates.