Salinity and K+ deficiency are two environmental constraints that generally occur simultaneously under field conditions, resulting in severe limitation of plant growth and productivity. The present study aimed at investigating the effects of salinity, either separately applied or in combination with K+ deficiency, on growth, photosynthetic performance, secondary metabolites content, and related antioxidant capacity in Sulla carnosa. Seedlings were grown hydroponically under sufficient (6000 μM) or low (60 μM) K+ supply with 100 mM NaCl (C + S and D + S treatments, respectively). Either alone or combined with K+ deficiency, salinity significantly restricted the plant growth. K+ deficiency further increased salt impact on the photosynthetic activity of S. carnosa, but this species displayed mechanisms that play a role in protecting photosynthetic machinery (including non photochemical quenching and antioxidant activity). In contrast to plants subjected to salt stress alone, higher accumulation of phenolic compounds was likely related to antioxidative defence mechanism in plants grown under combined effects of two stresses. As a whole, these data suggest that K+ deficiency increases the deleterious effects of salt stress. The quantitative and qualitative alteration of phenolic composition and the enhancement of related antioxidant capacity may be of crucial significance for S. carnosa plants growing under salinity and K+ deficient conditions.