The escalating demand for sustainable agriculture calls for innovative strategies that enhance crop resilience while minimizing dependence on synthetic fertilizers. This study evaluated the synergistic effects of a microbial consortium (PYS), organic fertilizer (OF), glycine (Gly), and indole-3-acetic acid (IAA) on lettuce under heat stress. The experiment was conducted in a greenhouse in Bangkok, Thailand, simulating tropical high-temperature conditions. The PYS+OF+Gly treatment significantly improved fresh weight, matching the performance of chemical fertilizer (CF) and indicating a strong growth-promoting synergy. Chlorophyll a, chlorophyll b, and carotenoid contents were higher in PYS or PYS+OF treatment, suggesting enhanced photosynthetic efficiency. At 60 days, PYS-based treatments also led to substantial increases in total phenolics and flavonoids, coupled with reduced lipid peroxidation and elevated antioxidant activities (DPPH, APX, CAT, POD, and SOD). However, vitamin C levels remained highest in the CF and OF controls, indicating a potential metabolic shift toward phenylpropanoid rather than ascorbate biosynthesis. Overall, our results demonstrate that combining microbial consortia with organic and biostimulant inputs could enhance growth, stress tolerance, and the nutritional quality of lettuce. This integrated approach presents a promising strategy for climate-resilient crop production and warrants further validation across different crops, environmental settings, and large-scale agricultural systems.