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Background: Salt stress seriously impacts the plantation and productivity of global alfalfa (Medicago sativa). Previously, we genetically defined the M. sativa DROUGHT-INDUCED UNKNOWN PROTEIN 1 (MsDIUP1) as a drought tolerance gene of alfalfa. However, whether it is associated with salt tolerance needs to be explored. Results: In this study, we re-classified MsDIUP1 as a putative novel late embryogenesis abundant protein, and found its promoter activity was strongly induced by salt and ABA treatment. Heterologous overexpression of MsDIUP1 in Arabidopsis led to enhanced salt tolerance and ABA insensitivity, with a higher fresh weight, lateral root number, primary root length, and survival rate compared to the wild-type (WT) plants. Similarly, homologous overexpression of MsDIUP1 also enhanced the salt tolerance of alfalfa over-expression (OE) plants, accompanied by reduced oxidative damage, higher K+/Na+ ratio, increased osmoprotectant accumulation, and improved photosynthetic capacity relative to the WT plants. In reverse, disruption of MsDIUP1 expression caused a salt-hypersensitive phenotype of alfalfa RNA interference (RNAi) plants, with higher oxidative damage, less osmoprotectant accumulation, and lower photosynthetic pigment content than that of the WT. These physiological discoveries were in accordance with differential transcript profiles of the relevant genes associated with stress signaling, antioxidant defense, and osmotic adjustment in the salt-treated alfalfa WT, OE, and RNAi lines. Furthermore, we found that alfalfa OE plants showed no apparent alteration in their forage quality compared with the WT plants. Conclusions: Overall, these results reveal that MsDIUP1 improves plant salt tolerance via regulating stress signaling, antioxidant defense, ion homeostasis, osmotic adjustment, and photosynthesis. [ABSTRACT FROM AUTHOR]