Objectives: This study aimed to determine the effect of nurse-based breastfeeding support during the immediate postpartum period on mothers’ breastfeeding self-efficacy levels. Methods: A quasi-experimental study was conducted with 256 mothers in the immediate postpartum period (128 in the intervention group who received nurse-based breastfeeding support and routine care and 128 in the control group who received routine care) during February–June 2018. Data were collected using the Mother-Infant Data Sheet, the Breastfeeding Self-Efficacy Scale-Short Form (BSES-SF), and the Nurse-Parent Support Tool (NPST). Results: BSES-SF and NPST mean scores of mothers in the intervention group were higher than those of mothers in the control group (p < 0.01). While the association between breastfeeding self-efficacy and nurse support was low in the control group (0.271), it was rather high in the intervention group (0.693) (p < 0.05). It was found that nurse support explained 48% of the variance in breastfeeding self-efficacy in the intervention group, but only 7.3% of the variance in breastfeeding self-efficacy in the control group. Conclusions for Practice: The results of the study indicate that nurse support for mothers in the immediate postpartum period based on breastfeeding training has a positive impact on breastfeeding self-efficacy.