Abstract: The gradual increases in the atmospheric CO₂ concentration and temperature are the two primary characteristics of global climate change. However, the rising atmospheric CO₂ concentration could alleviate the negative effects of heat stress. Therefore, the aim of this study was to explore the mitigating effects of elevated CO₂ on the growth and antioxidant system a perennial grass under heat stress. Tall fescue (Festuca arundinacea cv. ‘Barlexas’) was exposed to either ambient CO₂ concentration (400 μmol/mol) or elevated CO₂ concentration (800 μmol/mol) under optimal growth temperature (25/15 ℃ day/night) or elevated temperature (35/25 ℃ day/night) conditions. After 28 days of the experimental treatment, heat stress caused a significant reduction in relative growth rate (RGR), leaf net photosynthetic rate (P_n), number of green leaves, antioxidant enzyme activities [(superoxide dismutase, (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX)], as well as an increase in electrolyte leakage (EL), and the contents of malondialdehyde (MDA) and reactive oxygen species (ROS) (H₂O₂ and O₂^-·). Elevated CO₂ mitigated these negative effects of heat stress in tall fescue. Compared with plants grown under heat stress and ambient CO₂, those grown under heat stress and elevated CO₂ had lower leaf EL and MDA contents (72% and 39% lower, respectively); the P_n, F_v/F_m, and number of green leaves were increased by 1.74-fold, 17%, and 1.65-fold, respectively; and the ROS contents were significantly lower (H₂O₂ and O₂^-· contents decreased by 46% and 31%, respectively). These results demonstrated that elevated CO₂ can improve the heat tolerance of tall fescue through increased photosynthetic capacity, cellular membrane stability, and decreased ROS accumulation.