AbstractTo improve the ability of Cyphomandra betacea to accumulate selenium (Se), the effects of uniconazole (S3307) at different concentrations on the growth and Se accumulation of C. betacea seedlings were studied in this experiment. S3307 at different concentrations increased the biomass of C. betacea seedlings, and 20 mg L−1 S3307 had the best effect. The application of S3307 also increased the net photosynthetic rate, stomatal conductance, intercellular CO2 concentration, transpiration rate, photosynthetic pigment (chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid) content, and antioxidant enzyme (superoxide dismutase, catalase, and peroxidase) activity of C. betacea seedlings. Application of S3307 increased the Se content in the different organs of C. betacea seedlings to some extent. Besides, the different S3307 concentrations demonstrated a polynomial regression relationship with the root and shoot biomass and the root and shoot Se content. Besides, Se content of the various organs significantly positive correlated with the different organs' biomass. These results indicate that S3307 promotes C. betacea seedling growth and improves Se accumulation ability, with 20 mg L−1 demonstrating the highest response.