Plants acclimate to temperature by adjusting their photosynthetic capacity over weeks to months. However, most evidence for photosynthetic acclimation derives from leaf-scale experiments. Here, we address the scarcity of evidence for canopy-scale photosynthetic acclimation by examining the correlation between maximum photosynthetic rates (Amax,2000) and growth temperature ((T_air ) ̅) across a range of concurrent temperatures and canopy foliage quantity, using data from over 200 eddy covariance sites. We detect widespread thermal acclimation of canopy-scale photosynthesis, demonstrated by enhanced Amax,2000 under higher (T_air ) ̅, across flux sites with adequate water availability. A 14-day period is identified as the most relevant time scale for acclimation across all sites, with a range of 12–25 days for different plant functional types. The mean apparent thermal acclimation rate across all ecosystems is 0.41 (-0.38–1.04 for 5th–95th percentile range) µmol m-2 s-1 C-1, with croplands showing the largest and grasslands the lowest acclimation rates. Incorporating an optimality-based prediction of leaf photosynthetic capacities into a biochemical photosynthesis model is shown to improve the representation of thermal acclimation. Our results underscore the critical need for enhanced understanding and modelling of canopy-scale photosynthetic capacity to accurately predict plant responses to warmer growing seasons.