The relationship between maternal smoking and gestational diabetes mellitus (GDM) is inconclusive. We investigated whether prepregnancy smoking is a risk factor for insulin-requiring GDM in Korean women. Using the National Health Insurance Service database, 325,297 women who delivered between 2011 and 2015 and who received a health examination within 52 weeks before pregnancy were included. Insulin-requiring GDM was defined as no claims for diabetes mellitus and a fasting blood glucose level of < 126 mg/dL before pregnancy, and initiation of insulin treatment during pregnancy. Smoking status was identified in a self-reported questionnaire completed during the health examination. There were 2,114 women (0.65%) with GDM who required insulin therapy. Compared with nonsmokers, the fully adjusted odd ratios (ORs) of former smokers and current smokers for insulin-requiring GDM were 1.55 (95% confidence interval [CI] 1.27-1.90) and 1.73 (1.42-2.09), respectively. The ORs (95% CIs) of insulin-requiring GDM among women who reported ≤ 2, 2-≤ 4, 4-≤ 6, 6-≤ 8, 8-≤ 10, and > 10 pack-years of smoking were 1.50 (1.22-1.84), 1.71 (1.31-2.22), 1.60 (1.13-2.26), 1.97 (1.14-3.40), 2.34 (1.22-4.51), and 2.29 (1.25-4.22), respectively, compared with nonsmokers (P for trend < 0.001). This association was similar in women with or without obesity and abdominal obesity. In conclusions, women who smoke have a significantly higher risk of GDM requiring insulin therapy, which may be proportional to the cumulative exposure to smoking. Cessation of smoking should be emphasized in women of childbearing age for the prevention of GDM.