The rs10830963 variant of the Melatonin Receptor 1B (MTNR1B) gene is associated with the development of gestational diabetes mellitus (GDM). We hypothesized that carrying the rs10830963/G risk allele had effect on antenatal insulin therapy (AIT) initiation in GDM in a body mass index (BMI)-dependent manner. Design: In this post hoc analysis the MTNR1B rs10830963 genotype and the clinical data of 211 Caucasian GDM patients were assessed. As a first step, a pre-pregnancy BMI threshold was determined where the effect of MTNR1B rs10830963/G allele carrying on AIT initiation was the most significant using logistic regression. Maternal age adjusted real-life odds ratios (OR) values were calculated. The chi-square test was also used to calculate the p value and 10.000 bootstrap simulations were performed in each case to re-assess the statistical power and the OR. Carrying the MTNR1B rs10830963/G allele increased the odds of AIT initiation (OR = 5.2, p = 0.02 [χ2 test], statistical power = 0.53) in GDM patients with pre-pregnancy BMI ≥ 29 kg/m2. The statistical power reached 0.77, when the pre-pregnancy BMI cutoff of 27 kg/m2 was used and the genetic effect on AIT initiation was still significant, but only using the logistic regression model. Carrying the MTNR1B rs10830963/G risk allele—in interaction with pre-pregnancy BMI—is likely be considered as a candidate pharmacogenetic marker of antenatal insulin therapy initiation and should be further assessed in precision medicine trials in GDM.