Although plant photosynthetic capacity as determined by the maximum carboxylation rate (i.e., Vc,max25) and the maximum electron transport rate (i.e., Jmax25) at a reference temperature (generally 25 °C) is known to vary considerably in space and time in response to environmental conditions, it is typically parameterized in Earth system models (ESMs) with tabulated values associated with plant functional types. In this study, we have developed a mechanistic model of leaf utilization of nitrogen for assimilation (LUNA) to predict photosynthetic capacity at the global scale under different environmental conditions. We adopt an optimality hypothesis to nitrogen allocation among light capture, electron transport, carboxylation and respiration. The LUNA model is able to reasonably capture the measured spatial and temporal patterns of photosynthetic capacity as it explains ∼55% of the global variation in observed values of Vc,max25 and ∼65% of the variation in the observed values of Jmax25. Model simulations with LUNA under current and future climate conditions demonstrate that modeled values of Vc,max25 are most affected in high-latitude regions under future climates. ESMs that relate the values of Vc,max25 or Jmax25 to plant functional types only are likely to substantially overestimate future global photosynthesis.