The aim of this study was to find the optimal gibberellic acid (GA₃) and copper sulfate (CuSO₄·5H₂O) concentrations to reduce the number of seeds and increase the quality of Bac Son mandarin fruit. In experiment 1, the control plants (without GA₃) were sprayed with water, and the remaining plants were sprayed with different concentrations of GA₃ (50, 75, 100, or 125 ppm). In experiment 2, the control plants (without CuSO₄·5H₂O) were sprayed with water, and the remaining plants were sprayed with different concentrations of CuSO₄·5H₂O (50, 75, 100, or 125 ppm). Spraying GA₃ at 100 ppm in 2018 produced the lowest seed number and the highest theoretical yield. In 2019, spraying GA₃ led to a lower seed number and a higher actual yield compared with the control. Similarly, spraying CuSO₄·5H₂O significantly reduced the number of seeds/fruit and significantly increased the theoretical yield in 2018. In 2019, the total number of seeds/fruit was significantly lower in the CuSO₄·5H₂O treatments than in the control. Importantly, both GA₃ and CuSO₄·5H₂O treatments did not adversely affect the fruit’s biochemical parameters or yield. These findings demonstrate that spraying GA₃ or CuSO₄·5H₂O at a certain concentration can effectively reduce the number of seeds per fruit in Bac Son mandarin without compromising fruit quality or yield.