Introduction: The aim of this study is to reveal the factors affecting primiparous pregnant women’s breastfeeding self-efficacy levels. This is a descriptive correlational study. Method: The sample included 512 primiparous pregnant women. A personal information form, the Prenatal Breastfeeding Self-Efficacy Scale (PBSES), the Multidimensional Scale of Perceived Social Support (MSPSS) and the Pregnancy-Related Anxiety Questionnaire–Revised (PRAQ-R2) were used for data collection. Results: The pregnant women’s mean total PBSES scores were found to differ significantly based on their descriptive and obstetric characteristics. There was a positive correlation between their mean PBSES scores and their MSPSS total and subscale mean scores. Additionally, there was a negative correlation between the participants’ mean total PBSES scores and their mean total PRAQ-R2 and “concern about own appearance” subscale scores. Place of residence for the longest time, pregnancy risk group, folic acid use before pregnancy, perceived family support, perceived support from friends and concern about own appearance were significant factors in explaining the variance in the pregnant women’s PBSES total scores. Discussion: Both descriptive and obstetric characteristics of the primiparous pregnant women affected their breastfeeding self-efficacy levels. Social support was also determined to affect the participants’ breastfeeding self-efficacy levels positively, and whereas anxiety affected these levels negatively.