Background: Breast milk is a God-given gift that conveys a mother’s love and compassion and that is made according to the needs and age of the child. Mothers who are interested in the welfare of their newborns tend to breastfeed their children. Training programs have been shown to improve breastfeeding self-efficacy in terms of both duration and amount. Purpose: The aim of this study was to evaluate the effect of using the progressive muscle relaxation (PMR) technique on the self-efficacy of breastfeeding in mothers with preterm infants. Methods: A clinical trial approach was used. Sixtymothers with preterm infants were randomly assigned to either the intervention or control group. The clinical trial ran for a period of 2 months for both groups. At 24Y72 hours postpartum, the researcher used the Jacobson method to provide 30Y45 minutes of individual training to the intervention group participants on PMR. Under the Jacobson method, mothers contract the 16 groups of muscles until they experience the feeling of pressure and then relax these muscles. The tools used in this study were the standard questionnaire of Dennis breastfeeding self-efficacy, which was completed by the participants at baseline, at the end of the fourth week, and during the eighth week. Data were analyzed using SPSS software. Results: No significant difference was observed between the intervention and control groups in terms of demographic variables (p 9 .05). Independent t tests found no significant difference between the two groups (p = .45) in terms of mean score of maternal breastfeeding self-efficacy at pretest and significantly higher scores for the intervention group than the control group at both 4 (p = .001) and 8 (p G .001) weeks posttest. Furthermore, the analysis of variance test showed significant differences in the mean score of breastfeeding self-efficacy for the intervention group between pretest and the first posttest and between the first posttest and the second posttest, respectively (p G .001). Conclusions/Implications for Practice: Considering the effect of relaxation training on the breastfeeding self-efficacy of mothers with preterm infants, training and performing these exercises as an effective and low-cost method to improve the health of mothers, particularly mothers of preterm infants, are recommended. The PMR technique facilitates the self-efficacy of breastfeeding in mothers with preterm infants and should be considered as an effective strategy to improve nursing care and the provision of better support services for mothers who breastfeed their infants.