Breastfeeding is beneficial for mothers with gestational diabetes mellitus (GDM). Saudi Arabia is considered one of the countries with the highest prevalence of GDM. Mothers with GDM have a low intention to breastfeed and are less likely to continue breastfeeding. This study aimed to measure breastfeeding self-efficacy among expectant mothers with GDM and quantify its determinants. This cross-sectional study recruited expectant mothers with GDM from an antenatal care clinic and queried them on breastfeeding knowledge and attitudes using the Arabic validated prenatal breastfeeding self-efficacy scale (PBSES). The study took place at the Medical City of King Saud University, during January–April 2021. The average PBSES score among 145 GDM Saudi participants was 64.07 ± 16.3. Higher academic level, previous satisfactory breastfeeding experiences, breastfeeding intention, six months or more breastfeeding experience, and health education were significantly positively correlated with PBSES score. A higher knowledge score was also correlated with a higher PBSES score (p = 0.002). Longer breastfeeding duration (β.197, p = 0.036), satisfactory previous breastfeeding experience (β.218, p = 0.020), and higher knowledge score (β.259, p = 0.004) were significant predictors of a high PBSES score. Breastfeeding self-efficacy is low among expectant Saudi mothers with GDM, especially those with unsatisfactory previous experience or low knowledge scores. Establishing systematic education about breastfeeding during antenatal care is recommended to improve breastfeeding experience and improve GDM outcomes.