OBJECTIVE: To assess the effects of selenium supplementation on blood glucose levels in women with gestational diabetes mellitus (GDM). STUDY DESIGN: Randomised controlled trial. Place and Duration of the Study: Department of Internal Medicine, Istanbul Medipol University, Faculty of Medicine, Istanbul, Turkiye, from February to July 2023. METHODOLOGY: In the first phase of this study, the selenium levels of the pregnant women who routinely had an oral glucose tolerance test were measured, and in the second phase of the study, the pregnant women diagnosed with GDM were randomly divided into two groups that received 4-week interventions: Diet alone and diet plus selenium supplementation (200 µg/day). RESULTS: Selenium level in pregnant women with GDM was significantly lower than in healthy pregnant women, and a selenium level less than 80 ng/ml predicted GDM diagnosis with a sensitivity of 58.59% and a specificity of 67.11%. Pregnant women with low selenium (<80 ng/ml) had a 2.709-fold higher risk for GDM compared to those with higher values. Fasting blood glucose levels decreased significantly in both groups after the respective interventions, but the decrease was greater in selenium recipients. Furthermore, fasting, 1st and 2nd hour blood glucose levels were lower in selenium recipients compared to those who only received diet. CONCLUSION: Selenium level in pregnant women with GDM was low compared to healthy pregnant women. Selenium supplementation had a beneficial impact (compared to diet only) on blood glucose levels in pregnant women with GDM. KEY WORDS: Pregnancy, Pregnancy outcome, Diabetes, Gestational, Dietary supplements, Selenium.