Miscanthus is a class of C₄ perennial grasses, which can be cultivated on marginal land even with high salinity. However, the future environment may be altered by elevated atmospheric CO₂ concentration ([CO₂]) and knowledge is limited about the interactive impacts of CO₂ enrichment and salinity on this C₄ bioenergy crop. In this study, three Miscanthus genotypes (M. sacchariflorus, M. × giganteus, and M. lutarioriparius) were grown under either ambient (400 ppm) [CO₂] (a[CO₂]) or elevated (800 ppm) [CO₂] (e[CO₂]) at five salinity levels (0, 50, 100, 150, and 200 mm NaCl denoted as S0, S1, S2, S3, and S4, respectively), and the impacts of e[CO₂] on plant physiological responses to salt stress were investigated. Our results suggested that e[CO₂] had no obvious effect on net photosynthetic rate (A_n), but significantly reduced the stomatal conductance (g_s), thus improving water use efficiency regardless of salinity levels. In addition, e[CO₂] could improve water potential of plants under both control and saline conditions, but the magnitude of increase was highly genotypic dependent. The maximum quantum yield of photosystem II (F_v/F_m) was not altered by e[CO₂], which, however, could alleviate the negative effect of salt on F_v/F_m. Furthermore, salt stress increased the concentration of abscisic acid (ABA) in xylem sap and leaves, while the effect of e[CO₂] on ABA level was closely associated with genotypes. e[CO₂] reduced Na⁺ concentration and had positive influences on maintaining Na⁺/K⁺ ratio, thus favoring ionic homeostasis, although such effect was genotype dependent. Collectively, our data suggested that e[CO₂] could partially mitigate the detrimental effects of salinity, conferring higher salt tolerance of Miscanthus.