Gestational diabetes mellitus (GDM) is a common metabolic disease that occurs during pregnancy, with the placenta playing an important role in its pathophysiology. Currently, the role of galectin-9 in the development of GDM is unknown. The aim of this study was to compare galectin-9 concentrations in healthy pregnant women and those with GDM. Galectin-9 levels were assessed in serum samples taken both just before and after delivery, as well as in urine samples collected in the postpartum period. Maternal body composition and hydration status were evaluated using the bioelectrical impedance analysis (BIA) method. There were no statistically significant differences in the concentration of galectin-9 in women with GDM compared to healthy pregnant women in their serum samples taken just before delivery, nor in their serum and urine samples collected in the early postpartum period. However, serum galectin-9 concentrations taken before delivery were positively correlated with BMI and parameters related to the amount of adipose tissue assessed in the early postpartum period. Additionally, there was a correlation between serum galectin-9 concentrations taken before and after delivery. Galectin-9 is unlikely to become a diagnostic marker for GDM. However, this subject requires further clinical research in larger groups.