BACKGROUND: Despite the increasing rate of exclusive breastfeeding in Indonesia, there is still a need for supportive interventions. The breastfeeding self-efficacy of mothers is a key factor positively associated with optimum breastfeeding practices. Our analysis aims to assess the determinants of low breastfeeding self-efficacy amongst a sample of women with children aged under 6 months in Malang and Sidoarjo Districts, East Java, Indonesia. METHODS: We used information from 1210 mothers of children aged < 6 months recruited in the BADUTA study conducted in 2015-2016 in Malang and Sidoarjo Districts. The outcome variable in this analysis was mothers' self-efficacy for breastfeeding using the 14 statements in the Breastfeeding Self-Efficacy-Short Form. We evaluated 17 potential predictors of breastfeeding self-efficacy, organized into six sub-groups of variables: (1) context/demographic; (2) household factors; (3) maternal characteristics; (4) child characteristics; (5) breastfeeding practices; and (6) antenatal and delivery care. Logistic regression analyses were employed to examine factors associated with mothers' self-efficacy with breastfeeding. RESULTS: More than half of the women in this study had a low level of self-efficacy. One of the factors associated with low breastfeeding self-efficacy found in this study was mothers' problems related to breastfeeding. Mothers who had problems with breastfeeding not related to illness (adjusted odds ratio [aOR] 3.27; 95% CI 2.45, 4.36) or problems related to both illness and non-illness conditions (aOR 3.57; 95% CI 1.37, 9.33) had higher odds of low breastfeeding self-efficacy than those who did not have any problems. Compared to mothers who completed university education, there was a significantly higher odds of low breastfeeding self-efficacy in mothers who completed primary school or lower (aOR 1.88; 95% CI 1.16, 3.05); completed junior high school (aOR 2.27; 95% CI 1.42, 3.63); and completed senior high school (aOR 1.94; 95% CI 1.29, 2.91). Other significant predictors of low breastfeeding self-efficacy were mothers not exposed to any breastfeeding interventions (aOR 1.87; 95% CI 1.09, 3.22); working outside the house (aOR 1.69; 95% CI 1.23, 2.32); not obtaining any advice on breastfeeding (aOR 1.40; 95% CI 1.08, 1.82); with low knowledge of breastfeeding (aOR 1.38; 95% CI 1.03, 1.84); and delivered by Caesarean section (aOR 1.34; 95% CI 1.05, 1.70). CONCLUSIONS: Multipronged breastfeeding education programs and support are required to improve women's self-efficacy with breastfeeding. Improved access to breastfeeding counselors, active support for mothers following cesarean delivery, and increased supporting facilities at workplaces are essential to improve self-efficacy with breastfeeding.