Aim: This study aims to determine the predictors of antenatal insulin therapy (AIT) in patients with gestational diabetes mellitus (GDM) who applied to the outpatient endocrine clinic of a tertiary hospital. Material and Methods: The study included 619 patients with GDM.Oral glucose tolerance test (OGTT), HbA1c (%), lipid levels, and other biochemical measurements of two groups (patients with or without insulin use) were compared with Mann-Whitney U or Student t-test. Demographic characteristics, obesity, and diet adherence were compared with the Chi-Square test. OGTT-75 measurements between the three insulin groups (basal, bolus or both uses) were compared with ANOVA. Results: In this study, 27.0% of the GDM patients needed insulin therapy. The insulin group had a significantly higher rate of obese, morbidly obese, and poor diet adherence patients than the MNT group and had significantly higher values measured in the 75-g OGTT (OGTT-75) for 0-h glucose, 2-h glucose, HbA1C (%), and triglyceride. In OGTT-75, all three measurement values (0-h, 1-h, 2-h glucose) above the cut-off were associated with insulin requirement. In OGTT-75, 2-h glucose value > 151.5 mg/dL (68.4% sensitivity and 60.0% specificity) predicted general need for insulin, and 0-h glucose value > 95.0 mg/dL (72.0% sensitivity and 69.0% specificity) predicted the need for basal AIT. Conclusion: We suggest identifying the risky group for insulin therapy at the time of diagnosis in GDM. Closer follow-up is required for the patients found to be in the risk group for insulin therapy.