Background/Objective Because galectin-3 has been proposed to regulate obesity and insulin resistance in mice, we hypothesized that circulating galectin-3 levels are associated with presence of gestational diabetes mellitus (GDM), progesterone, and insulin resistance. Methods Circulating galectin-3 levels were measured using an enzyme-linked immunosorbent assay (ELISA) in women with GDM (n = 137) and their controls (n = 81). Associations of galectin-3 and progesterone with GDM and insulin resistance were evaluated using regression models. Results Circulating galectin-3 levels were increased in the individuals with GDM (P < .001) and associated significantly with progesterone (r = 0.42,P < .001), gestational age at sampling (r = 0.23,P < .001), current body mass index (BMI; r = 0.17,P= .02), estrogen (r = 0.15,P < .03), fasting glucose (r = 0.41,P < .001), fasting insulin (r = 0.39,P < .001), and homeostasis model assessment of insulin resistance (HOMA-IR; r = 0.44,P < .001). After adjustment for potential confounders, including current BMI, subjects in the highest tertile of galectin-3 levels were more likely to have GDM (odds ratio 4.71, 95% confidence interval 2.01-11.06) as compared with the lowest tertile. The association between circulating galectin-3 levels and GDM remained significant after adjusting for progesterone, but significantly attenuated after adjustment with HOMA-IR. Furthermore, the multiple linear regression analyses after adjustment for confounders showed an independent association between galectin-3 levels and HOMA-IR (beta = .41,P < .001), suggesting that association of circulating gelactin-3 levels with GDM might be mediated via insulin resistance. Progesterone demonstrated the expected associations with galectin-3, GDM, and HOMA-IR. Conclusions Circulating galectin-3 levels are associated with GDM possibly through increased insulin resistance. The association of galectin-3 with progesterone highlights a potential role of progesterone in its interaction with galectin-3.