Abstract: A comprehensive understanding of the photosynthetic response of wetland plants to increased atmospheric CO₂ concentrations is crucial for predicting the effects of future climate change on the structure and function of wetland ecosystems. This study used the open top chamber systems (OTCs) to simulate two CO₂ concentrations (ambient and +400 μmol/mol) in coastal wetlands of the Yellow River Delta. The intact soil columns (100 cm height and 40 cm diameter), along with C3 (Suaeda salsa) and C4 (Spartina alterniflora) plants, were extracted from wetland using stainless steel barrel and transferred into the OTCs. To investigate the impacts of elevated atmospheric CO₂ concentrations on the photosynthetic characteristics of the two wetland plants, the Li-6800 portable photosynthesis system was employed to measure the net photosynthesis rate, stomatal conductance, transpiration rate, light and CO₂ response curves. Although elevated CO₂ concentrations exerted a stronger stimulatory effect on photosynthetic activity in the C3 plant S. salsa compared to the C4 plant S. alterniflora, the net photosynthetic rate of S. salsa remained significantly lower than that of S. alterniflora under both ambient and elevated CO₂ conditions. The elevated CO₂ concentration significantly impacted the parameters of the light response curves for both plants. The maximum net photosynthetic rate of S. salsa displayed a more robust response to the increased CO₂ concentration compared to S. alterniflora. Compared to ambient CO₂ levels, the turning point of CO₂ response curve for S. salsa under elevated CO₂ shifted backward, while that of S. alterniflora showed little change. Notably, elevated CO₂ led to a decrease in stomatal conductance and transpiration rate in S. alterniflora, but not for S. salsa. Overall, elevated CO₂ concentration significantly influenced the photosynthetic traits of both plant species, with S. salsa demonstrating a greater capacity to adapt to elevated CO₂ concentrations. This research provided insights for evaluating the effects of increased CO₂ concentrations on carbon cycles in wetland ecosystems.