Photosynthetic functions are key traits in determining the carbon uptake of urban trees. It is essential to clarify the seasonal dynamics of photosynthesis to evaluate the CO₂ uptake in urban areas. We investigated seasonal variations in the photosynthetic traits of Ginkgo biloba, which is a major urban deciduous tall tree in Japan. Seasonal variations in the leaf photosynthetic traits were well fitted to quadratic models, in which they peaked around the summer solstice and then declined with time. Seasonal variations in the environmental variables, such as photoperiod, temperature, and solar radiation, were compared to those of the leaf photosynthetic traits, in which the photoperiod explained well variations in the leaf photosynthetic traits. Seasonal variations in photosynthesis were largely governed by non-stomatal limitations, i.e., mesophyll and biochemical limitations. The high synchrony of the photoperiod and photosynthetic traits during leaf maturation may cause an enhancement in the daily carbon uptake of G. biloba leaves around the summer solstice, which has the longest photoperiod, and thus, will lead to an increase in the annual carbon uptake. These results indicate the importance of photoperiod incorporation to predict the effect of future warming on the production of urban tree species.