Light is a crucial environmental regulator for Tricholoma giganteum (T. giganteum). This study investigated the effects of light quality and photoperiod on its growth, physiology, and nutritional composition. During the mycelial stage, blue light (BL) exposure for 5 d promoted the highest growth rate (0.74 mm d⁻¹, 45% higher than dark control, p < 0.05). Red light (RL) enhanced antioxidant capacity, elevating superoxide dismutase (SOD) activity to 240.20 U·mL⁻¹ (after 5 d) and DPPH radical-scavenging activity to 276.11% (after 3 d). Ultraviolet (UV) suppressed polyphenol oxidase (PPO) activity. BL also increased mycelial polysaccharide content (6.45 g·100 g⁻¹). In the fruiting stage, green light (GL) improved agronomic traits and first-grade yield (3.75 kg), while also promoting the accumulation of glutamate (4.39 g·100 g⁻¹), a key umami compound. Further photoperiod optimization revealed that 4 h of daily GL exposure shortened the fruiting cycle, achieved the highest biological efficiency (98.4%), and maximized both polysaccharide (38.17 g·100 g⁻¹) and glutamate contents (5.70 g·100 g⁻¹). These results recommend a two-stage lighting protocol: BL for mycelial growth and a 4 h daily GL for fruiting, providing a scientific basis for the industrial cultivation of T. giganteum.