Temperature response of gross primary productivity (GPP) is a well-known property of ecosystem, but GPP at the optimum temperature (GPP_Topt) has not been fully discussed. Our understanding of how GPP_Topt responds to warming and water availability is highly limited. In this study, we analyzed data at 326 globally distributed eddy covariance sites (79oN-37oS), to identify controlling factors of GPP_Topt. Although GPP_Topt was significantly influenced by soil moisture, global solar radiation, mean annual temperature, and vapor pressure deficit in a non-linear pattern (R2 = 0.47), the direction and magnitude of these climate variables’ effects on GPP_Topt depend on the dryness index (DI), a ratio of potential evapotranspiration to precipitation. The spatial pattern showed that soil moisture did not affect GPP_Topt across energy-limited sites with DI < 1 while dominated GPP_Topt across water-limited sites with DI >1. The temporal pattern showed that GPP_Topt was lowered by warming or low precipitation in water-limited sites while energy-limited sites tended to maintain a stable GPP_Topt regardless of changes in air temperature. Vegetation types in humid climates tended to have higher GPP_Topt and were more likely to benefit from a warmer climate since it was not restricted by water conditions. This study highlights that the response of GPP_Topt to global warming depends on the dryness conditions, which explains the nonlinear control of water and temperature over GPP_Topt. Our finding is essential to realistic prediction of terrestrial carbon uptake under future climate and vegetation conditions.