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Light spectrum plays a crucial role in regulating the growth of hemp ( Cannabis sativa L.) plants and the biosynthesis of secondary metabolites. Several studies have demonstrated that additional red-light exposure increases biomass accumulation, while supplementary UV-A light stimulates cannabinoid synthesis. Nevertheless, the potential of stage-specific supplementation of red and UV-A light remains underexplored in its capacity to optimize cannabinoid yield in indoor hemp cultivation. In the present study, the effect of red light in combination with UV-A light on hemp biomass and cannabinoid accumulation was investigated using a high-CBD strain. There were four treatments: (1) white light throughout the growth period (control; V W R W ); (2) red light supplementation during the vegetative stage (V WR R W ); (3) UV-A supplementation (V W R WUV ) during the flowering stage; and (4) combined red and UV-A supplementation (V WR R WUV ) during the vegetative and flowering stages. Results showed that V WR R W promoted the number of effective branches (increased by 18.0%) compared to the control (V W R W ), resulting in an increase in inflorescence yield by 17.9%. V W R WUV increased CBG and CBD content by 52.7% and 12.1%, respectively, relative to the control. The effect of V WR R WUV on biomass and cannabinoid accumulation was the strongest among the treatments, with CBG and CBD yields reaching 0.53 g and 4.62 g per plant, representing significant increase of 91.8% ( p  < 0.01) and 44.1% ( p  < 0.01), respectively, compared to the control. However, there were no significant differences in CBD yield among the V WR R W , V W R WUV and V WR R WUV treatments, indicating that the combined supplementation of red and UV-A light did not have an additive effect on CBD accumulation. These findings highlight the potential of stage-specific spectral strategy to optimize both plant growth and phytochemical quantity.