A meta-analysis of the association of iron overload with gestational diabetes mellitus (GDM) may inform the health debate. We performed a meta-analysis investigating the association of iron biomarkers and dietary iron exposure with GDM. We identified 33 eligible studies (N = 44,110) published in 2001-2017. The standardized mean differences (SMD) in women who had GDM compared to pregnant women without were 0.25 mu g/dL (95% CI: 0.001-0.50) for iron, 1.54 ng/mL (0.56-2.53) for ferritin, 1.05% (0.02 to 2.08) for transferrin saturation, and 0.81 g/dL (0.40-1.22) for hemoglobin. Adjusted odds ratio for GDM were 1.58 (95% CI: 1.20-2.08) for ferritin, 1.30 (1.01-1.67) for hemoglobin, and 1.48 (1.29-1.69) for dietary heme intake. We did not find any differences in TIBC or transferrin concentration in women with and without GDM. We also did not find any association of increased transferrin receptor or increased intake of total dietary iron, non-heme iron or supplemental iron, with increased odds ratios for GDM. Considerable heterogeneity was present among the studies (0-99%), but no evidence of publication bias. Accumulating evidence suggests that circulating and dietary iron biomarkers among pregnant women are associated with GDM, but the results should be interpreted with caution due to the high heterogeneity of analyses. Randomized trials investigating the benefits of iron reduction in women at high risk for GDM are warranted.