Climate change is one of the major threats to global agricultural production and food security, particularly for crops such as summer squash (Cucurbita pepo L.), which are highly susceptible to heat, drought, and other environmental stresses. This study was conducted to investigate the effects of foliar application of humic acid on growth, yield, water use efficiency, and physiological responses of summer squash under drought stress in a field experiment. The experimental treatments consisted of three irrigation levels (100, 80, and 60 % ETc) and three humic acid concentrations (0, 150, and 300 mg L⁻¹). Water deficit stress significantly reduced vegetative growth, total fruit yield, and water use efficiency, while increasing proline accumulation and the activities of antioxidant enzymes. Foliar application of humic acid, particularly at 300 mg L⁻¹, alleviated the adverse effects of drought stress and improved plant growth, yield, and physiological performance. The highest leaf area (969.78 cm²), total fruit yield (32,950 kg ha⁻¹), chlorophyll content (2.27 mg g⁻¹ fresh weight), water use efficiency (8.17 kg m⁻³), and membrane stability index (84.5%) were obtained under full irrigation (100 ETc%) combined with foliar application of 300 mg L⁻¹ humic acid. In contrast, the highest proline content (9.68 mg 100 g⁻¹ FW) and the greatest activities of peroxidase and catalase were observed in plants treated with 300 mg L⁻¹ humic acid under 60 % ETc water deficit stress. Overall, the results indicate that foliar application of humic acid at 300 mg L⁻¹ is an effective strategy for improving growth, yield, water use efficiency, and drought tolerance in summer squash under both water-deficit and well-watered conditions.