Salinity severely reduced crop productivity. In this work, we study the effectiveness of Spirulina platensis (100 mg/l) as a foliar growth stimulator and its ability to reduce the harmful effect of salinity on Phaseolus vulgaris L. plants. The experiment was divided into four groups as follows: G1 (control), G2 (200 mM NaCl), G3 (100 mg/l, Spirulina platensis), and G4 (200 mM NaCl + 100 mg/l, Spirulina platensis). Salinity treatment (G2) showed that the plant height, shoot weight (fresh and dry), and number of leaves/plant decreased significantly during the vegetative growth stage; the number of pods/plant, number of seeds/pod, and pod weight (fresh and dry) at the time of harvest; and the contents of carotenoids, chlorophyll a+b, 100-seed weights, photosynthetic activity (assimilation of 14CO2), and total protein content also decreased significantly. The above parameters were enhanced by G4 treatment when compared to salt-stressed plants. Salinity treatment considerably increased the levels of superoxide dismutase (SOD), peroxidase activity (POD), catalase (CAT), glutathione peroxidase (GPX), malondialdehyde (MDA), free proline, and total phenol by 24.3, 32.2, 45, 18.6, 56.5, 38.4, and 13.9% respectively, as compared to the control treatment (G1). However, G4 treatment (200 mM NaCl + S. platensis, 100 mg/l) showed a significant decrease in SOD, POD, CAT, GPX, MDA, free proline, and total phenol by 2.4, 4.5, 4.5, 9.5, 20, 9.4, and 2.7%, respectively, as compared to the control plant (G1). Additionally, salinity dramatically reduced total protein by 36% as compared to the control. The G4 treatment (200 mM NaCl + S. platensis, 100 mg/l), on the other hand, increased total protein content by 31.3% when compared to the salinity treatment (G2). The salinity treatment (G2) decreased significantly the macronutrient content of N, P, and K by 26.1, 34.7, and 43.7%, respectively, and on the contrary, increased significantly the Na+ and Cl− levels by 55.8 and 23.1%, respectively, compared to the control plant (G1). In contrast to salt-stressed plants, G3 (S. platensis, 100 mg/l) and G4 (200 mM NaCl + S. platensis, 100 mg/l) applications considerably enhanced the macronutrient content and reduced the Na+ and Cl− concentrations. Obtained results proved that foliar applications of S. platensis at 100 mg/l have a high potential for improving growth, photosynthetic capacity, yield production, decreasing ROS-induced oxidative damage, and reducing DNA damage in salt-stressed Phaseolus vulgaris.