Background More than 550,000 late preterm infants are born each year in Indonesia. These infants, born between 340/7–366/7 weeks, have more complications than term infants. Breastfeeding is considered the most optimal nutrition for newborn infants. Two groups of factors are important for successful breastfeeding: infant and maternal factors. The infant factors can be evaluated using the Infant Breastfeeding Assessment Tool and the maternal aspects with the Breastfeeding Self-Efficacy Scale–Short Form. Aim To determine whether the Infant Breastfeeding Assessment Tool or the Breastfeeding Self-Efficacy Scale–Short Form was more predictive of successful breastfeeding among late preterm infants. Methods This study was conducted in the Academic Teaching Hospital in Surabaya, Indonesia in March–July 2017. Mothers who delivered their infant between a gestational age of 340/7 and 366/7 weeks were included. Results Fifty-four single born participant mother–infant pairs were included. The mean total Breastfeeding Self-Efficacy Scale–Short Form score was 57.8 ( SD = 8.9). The mean Infant Breastfeeding Assessment Tool score was 8.3 ( SD = 1.8). There was a significant correlation between the total Breastfeeding Self-Efficacy Scale–Short Form score and the Infant Breastfeeding Assessment Tool score ( p = .020, r = 0.316). The Breastfeeding Self-Efficacy Scale–Short Form was significantly higher in the participant (mothers) of the infants breastfed ≥ 4 months, compared to < 4 months, 61.59 ( SD = 5.78) versus 51.78 ( SD = 11.64; p = .001). No correlation was found between the duration of breastfeeding and Infant Breastfeeding Assessment Tool score ( p = .087) Conclusion Maternal factors were more important for successful breastfeeding in these late preterm infants than infant factors in our sample.