We aimed to verify effects of a self-efficacy-based breastfeeding support program (SBP) through a 1:1 face-to-face education and non-face-to-face education via phone counselling on breastfeeding and postnatal depression in primiparous women. This is a nonequivalent control group pre-posttest design study and primiparous women who gave birth at a tertiary general hospital in South Korea from May to Oct 2022 participated. Participants in the experimental group were provided with SBP (6 educational sessions) by the researcher for 4 wk after birth. Pre and post-test measures included breastfeeding knowledge, breastfeeding self-efficacy, adaption, breastfeeding rate, and postnatal depression. The participants were 48, with 26 in the SBP experimental group and 22 in the control group. Breast-feeding knowledge, breastfeeding self-efficacy, and adaption scores level of increase were significantly higher in the experimental group than in the control group (<.001, =.010, and =.012). Breastfeeding rate was significantly higher in the experimental group than in the control group (=.011). The score of postnatal depression was significantly lower in the experimental group than in the control group (=.032). The SBP was validated as a program that could promote the continuation of self-directed breast-feeding even after discharge by primiparous women experiencing breastfeeding for the first time and ultimately improve the health of both mother and infant, thereby providing evidence for effective breastfeeding intervention strategies for nursing in practice.