Introduction Exclusive breastfeeding is essential to reduce infant morbidity and mortality, as it meets the entire child's nutritional and immunological needs, strengthens the emotional bond between the mother-baby pair and protects against chronic and infectious diseases. Several factors interfere in this practice, among which the woman's self-efficacy stands out because such attitude represents maternal confidence in her ability to breastfeed her child. Objectives Evaluating self-efficacy of breastfeeding among women assisted in primary health care units and; verifying the association between socioeconomic and obstetric factors, and between the duration of exclusive breastfeeding and breastfeeding self-efficacy. Methods Cross-sectional study carried out with 216 mothers of children under six months, registered in four basic health units in the city of Rio de Janeiro, Brazil. The research was approved by the Ethics and Research Committees of the participating institutions. The Breastfeeding Self-Efficacy Scale - Short Form was used to measure the self-efficacy scores, and the Statistical Package for the Social Science Program was used to analyze such data. Results high levels of breastfeeding self-efficacy were associated with six or more prenatal consultations (OR = 0.92, 95% CI 0.84-1.01, p = 0.07), normal birth (OR = 1,25, 95% CI 1.07-1.45, p < 0.001), breastfeeding in the first half hour after delivery (OR = 1,17, 95% CI 1.08 - 1.27, p < 0.001) and the practice of exclusive breastfeeding for 6 months (OR = 1,86, 95% CI 1.27-2.27, p < 0.001). Conclusions prenatal care, vaginal delivery, breastfeeding in the first half hour of life and the practice of exclusive breastfeeding were related to the high and medium level of breastfeeding self-efficacy. The knowledge of the self-efficacy degree and the determining factors for the breastfeeding enable health professionals to implement effective interventions for promoting, protecting and supporting breastfeeding Key messages Maternal self-efficacy contributes to prevent early weaning and, thus, to reduce infant morbidity and mortality. Awareness of the determining factors of breastfeeding enables the implementation of effective interventions to promote and support breastfeeding.