Objective <p>Recent studies have demonstrated that serum ferritin levels were associated with gestational diabetes mellitus (GDM) in singleton pregnancies. However, this association has not been investigated in twin pregnancies. This study aimed to evaluate the association between serum ferritin levels and the risk of GDM in twin pregnancies.</p> Methods <p>This retrospective cohort study included 1828 women with twin pregnancies who delivered live births between July 2018 and June 2022. Serum ferritin levels were measured in the first trimester (6–14 gestational weeks). Multivariate logistic regression models were applied to evaluate the associations between serum ferritin levels and the risk of GDM. We also estimated the non-linear relationship of continuous serum ferritin and OR of GDM through linear mixed regression model with restricted cubic splines (RCS).</p> Results <p>There were 19.25% (352/1828) twin pregnant women developed GDM in this study. The median (interquartile range, IQR) serum ferritin concentration was 51.95 (30.95–89.25) ng/ml. When setting the lowest quartile of serum ferritin as a reference, the adjusted odds ratio(aOR) (95% confidence interval (CI)) for GDM was 1.95(1.39–2.74) in the highest serum ferritin quartile. A linear relationship between serum ferritin and GDM risk was observed (P for nonlinearity = 0.257). In addition, stratified analyses showed high serum ferritin levels increased the risk of GDM in women with pre-pregnancy body mass index (BMI)&lt;24&#xa0;kg/m² (Q3:aOR = 1.54,95% CI:1.04–2.29;Q4:aOR = 1.97,95% CI:1.34–2.91).</p> Conclusions <p>The retrospective study suggested that elevated serum ferritin level in the first trimester was associated with an increased risk of GDM in twin pregnancies, especially among women with BMI &lt; 24&#xa0;kg/m².</p>