This study examined the effects of supplemental blue and UV-A light on the growth, phytochemical composition, and essential oil (EO) production of peppermint (Mentha × piperita L.) under greenhouse conditions. Plants were exposed to natural light (control), UV-A, blue light, or a combination of UV-A and blue light using LEDs. Blue light increased leaf area (391.78 mm²) and shoot fresh weight (48.78 g), while the combination of UV-A and blue light boosted total chlorophyll (2.03 mg.g⁻¹ FW) and carotenoid content (0.46 mg.g⁻¹ FW). UV-A light enhanced antioxidant capacity (38.34% inhibition) and flavonoid content (32.78 mg.g⁻¹ DW) but reduced leaf area (253.42 mm²) and shoot fresh weight (33.35 g). Blue light yielded the highest EO content (1.84% w/w), while the combined UV-A+Blue light treatment resulted in the lowest (1.13% w/w). GC-MS analysis revealed that blue light increased menthone (18.62-24.30%) and menthofuran (2.32-2.63%) but reduced menthol levels, with the highest menthol content in the control (41.20%) and combined treatments (37.45%). Positive correlations were found between antioxidant activity, menthone, carotenoids, and flavonoids (r² > 0.50), highlighting their roles in stress response and secondary metabolism. These findings suggest that spectral manipulation can optimize peppermint growth and phytochemical composition. Blue light enhances biomass and EO yield, while UV-A improves antioxidant capacity and secondary metabolites. However, combining UV-A and blue light may reduce EO yield, underscoring the need for tailored light regimes to balance yield and quality in peppermint cultivation.