Background To examine possible markers for undiagnosed type 2 diabetes in early‐onset gestational diabetes mellitus (GDM). Methods A population‐based retrospective cohort study of women with GDM diagnosed prior to the 24th week, who delivered between 2005 and 2018, was conducted. The results of the 50‐g glucose challenge test, 100‐g oral glucose tolerance test, and the first available glycosylated hemoglobin (HbA1c) were examined as markers for type 2 diabetes diagnosed 6 to 12 weeks post pregnancy, which is suggestive of pregestational diabetes. This was done by calculating the area under the curve (AUC) of the receiver operating characteristic (ROC) curve for each marker. These markers and additional ones were evaluated for independency by stepwise multivariate logistic regression. Results Of 69 417 pregnancies, 3321 (5%) pregnancies were with GDM. Of them, in 261 (8%) women GDM was diagnosed prior to the 24th week, and 219 (84%) women had available data for analysis. Among them, 11 (5%) women had type 2 diabetes post pregnancy. Elevated HbA1c was the most accurate marker for undiagnosed type 2 diabetes (AUC 91% with 95% CI [81%‐100%]) and the only independent marker for undiagnosed type 2 diabetes in multivariate analysis (odds ratio [OR] 19; 95% CI [3.2‐109], C statistic = 88%). Using the ROC curve, a cutoff of 5.8% predicted type 2 diabetes with 89% sensitivity, 86% specificity, 99.4% negative predictive value, and 23% positive predictive value. Conclusions Early‐onset GDM with HbA1c ≥ 5.8% should be managed as type 2 diabetes. Definitive diagnosis should be done after delivery.