Abstarct: Abstract Drought stress, as the most critical abiotic stress, disrupts physiological and biochemical processes, leading to reduced growth and yield of lentil in arid and semi-arid regions. To enhance plant tolerance to drought, the use of growth-regulating compounds such as melatonin and spermidine, along with inoculation with plant growth-promoting rhizobacteria, represents a novel and effective approach. This study was conducted to evaluate the main and interactive effects of melatonin (0, 50, and 100 µM), spermidine (0, 0.5, and 1 mM), and inoculation with Pseudomonas fluorescens on physiological, biochemical, and yield-related traits of lentil (Bilesavar) during the 2023–2024 cropping seasons under rainfed conditions in the Chahardangeh region of Sari County. The results showed that the combined application of 100 µM melatonin and 1 mM spermidine, in the absence of bacterial inoculation, significantly increased leaf relative water content, total dry biomass, and grain yield compared with the control. Moreover, total chlorophyll and carotenoid contents, as well as the activities of antioxidant enzymes catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD), were enhanced, whereas oxidative stress indices, including malondialdehyde (MDA) and H₂O₂, were reduced. Stepwise regression and path analysis revealed that hundred-seed weight, number of seeds per plant, and stem dry weight had the greatest positive direct effects on grain yield. In contrast, leaf dry weight, despite its positive correlation with yield, exhibited a negative direct effect, indicating its indirect influence through other traits. Among physiological parameters, soluble leaf sugar content, total chlorophyll, carotenoids, and peroxidase activity were positively correlated with yield, while total phenols, flavonoids, and MDA showed negative associations. Overall, among the 18 treatments evaluated, the top three treatments were: (1) the combined application of 100 µM melatonin and 1 mM spermidine without bacterial inoculation, (2) inoculation with Pseudomonas fluorescens alone, and (3) application of 100 µM melatonin alone. Each of these treatments positively affected specific traits—such as increasing relative water content, chlorophyll and carotenoid concentrations, antioxidant enzyme activities, and yield components—thereby mitigating the adverse effects of drought stress and significantly improving lentil grain yield. Furthermore, the relationships among biochemical and physiological traits provide valuable insights for identifying key indicators contributing to yield improvement under water-deficit conditions.