Water stress has a negative impact on the yield and growth of crops worldwide and consequently has a global impact on food security. Many biochemical changes occur in plants as a response to water stress, such as activation of antioxidant systems. Molybdenum (Mo) plays an important part in activating the expression of many enzymes, such as CAT, POD, and SOD, as well as increasing the proline content. Mo therefore supports the defence system in plants and plays an important role in the defence system of mung bean plants growing under water stress conditions. Four concentrations of Mo (0, 15, 30, and 45 mg·L−1) were applied to plants, using two approaches: (a) seed soaking and (b) foliar application. Mung bean plants were subjected to three irrigation intervals (4 days control, 8 days-moderate water stress, and 12 days severe water stress). Irrigation intervals caused a reduction in the growth and production of mung beans, especially when the plants were irrigated every 12 days. It also led to the accumulation of malondialdehyde (MDA) and hydrogen peroxide (H2O2) in mung bean leaves, and these are considered to be indicators of lipid peroxidation and Reactive Oxygen Species (ROS) accumulation, respectively. On the other hand, applying Mo enhanced some growth and yield traits and also enhanced the defence system by upregulating antioxidant expressions, such as proline, catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD). The MDA content did not change under the effect of Mo treatments. However, H2O2 content slightly increased with an increase of Mo concentration of up to 30 mg·L−1 followed by a significant decrease when Mo concentration was increased to 45 mg·L−1. It can be concluded that Mo is a robust tool for the activation of the defence system in mung beans.