Terrestrial photosynthesis, or gross primary production (GPP), is the largest carbon f lux in the biosphere but its global magnitude and spatiotemporal dynamics remain uncertain. The global annual mean GPP is historically thought to be around 120 PgC yr-1, which is ~30-50 PgC yr-1 lower than GPP inferred from the oxygen-18 isotope (18O) and soil respiration. This disparity is a source of uncertainty in predicting climate–carbon cycle feedbacks. Here we infer GPP from carbonyl sulfide (OCS), an innovative tracer for CO2 diffusion from ambient air to leaf chloroplasts through stomata and mesophyll layers. We demonstrate that explicitly representing mesophyll diffusion is important for accurately quantifying the spatiotemporal dynamics of plant OCS uptake. From the estimated plant OCS uptake, we infer a global contemporary GPP of 157 (±8.5) PgC yr-1, which is consistent with estimates from 18O (150-175 PgC yr-1) and soil respiration (149_(-23; +29) PgC yr-1), but with an improved confidence level. Our global GPP is higher than satellite optical observation-driven estimates (120~140 PgC yr–1) that are used for Earth System Model benchmarking. This difference predominantly occurs in the pan-tropical rainforests and is corroborated by ground measurements, suggesting a more productive tropics than satellite-based GPP products indicated. As GPP is a primary determinant of terrestrial carbon sinks and may shape climate trajectories, our findings lay a physiological foundation on which the understanding and prediction of carbon–climate feedbacks can be advanced.