Abstract: Abstract: Since the beginning of industrial revolution, atmospheric CO2 concentration has increased from 310 μmol/mol to approximately 400 μmol/mol, and is expected to double at the end of the 21st century. Soil salinity is a prime abiotic stress that limits agriculture productivity worldwide. A pot experiment was conducted in climate chambers to investigate the response of quinoa (Chenopodium quinoa Willd) to salt stress under ambient (400 μmol/mol) and elevated (800 μmol/mol) atmospheric CO2 concentration. In this study, seeds of quinoa were surface sterilized and sown in peat-filled plastic trays in a greenhouse at ambient temperature. Four weeks after sowing, one seedling was transplanted to each plastic pot (the height of 25 cm and the diameter of 20 cm) containing 5 kg mixture of peat and sand and then placed in climate chambers with different CO2 concentrations. Plants at the five-leaf stage were irrigated with 0 mmol/L NaCl (non-saline) and 400 mmol/L NaCl (saline) solutions, respectively. During the experiment, leaf photosynthesis, stomatal conductance, intrinsic water use efficiency, ions concentrations, water potential, osmotic potential and pressure potential were determined. In addition, the plant height, shoot biomass, root biomass, 100-grain weight and grain yield were measured. The results showed that leaf photosynthesis rate and stomatal conductance were significantly reduced while intrinsic water use efficiency was significantly increased under salinity stress compared with control treatment without NaCl addition. The elevated atmospheric CO2 concentration enhanced stomatal conductance but decreased photosynthesis rate. In addition, the elevated atmospheric CO2 concentration significantly mitigated the negative effects of salt stress on quinoa and increased photosynthesis rate by 39.4% accompanied by decreasing stomatal conductance by 11.5% resulting in enhancing plant height, shoot biomass, root biomass, 100-grain weight and grain yield of quinoa by 8%, 20%, 82%, 19% and 34%, respectively. However, the fluctuation of quinoa photosynthesis rate and stomatal conductance gradually decreased with quinoa growth, resulting in photosynthetic acclimation. The rapid development of the root system under elevated atmospheric CO2 concentrations greatly increased the root-to-shoot ratio. Thus, it could improve the plant water absorption capacity and mitigate the dehydration of leaf cells induced by salt stress. Moreover, elevated atmospheric CO2 concentrations could regulate the hormone levels in plants and thus affect Na+ uptake and adjust water balance in cells. The significant difference wasn't found in leaf potential and osmotic potential under both ambient and elevated atmospheric CO2 concentration with non-saline irrigation at the same sampling time. However, elevated atmospheric CO2 concentrations significantly decreased leaf potential and osmotic potential than ambient atmospheric CO2 concentration with saline irrigation. This phenomenon effectively maintained pressure potential and alleviated water shortage in plant cells. Additionally, compared with ambient atmospheric CO2 concentrations, elevated atmospheric CO2 concentrations significantly improved plant water relations, decreased Na+ by 42% and improved K+ retention by 26% under salinity stress. Improved growth, physiology and yield performance were linked with better plant water (osmotic and water potential) and gas relations (leaf photosynthesis rate, stomatal conductance), low Na+ and high K+ contents in leaves. The results are helpful for understanding the physiological mechanism of salt tolerance in quinoa under elevated atmospheric CO2 concentrations. In addition, these findings also provide information for dealing with soil salinization, maintaining ecosystem stability and ensuring food security under the background of elevated atmospheric CO2.