This study was conducted to determine the relationship of breastfeeding self-efficacy with relationship satisfaction and family function in mothers during the first year postpartum. The study was carried out between 01 November 2021 and 01 January 2022 in the paediatric outpatient clinic of the Medical Faculty Hospital of a province in the Central Anatolian Region of Turkey. Responses from a total of 331 mothers who were within the first year postpartum were analyzed. The mothers' mean breastfeeding self-efficacy score was 55.3 (SD 9.1), and their effective breastfeeding status was found to be moderate. The following parameters were risk factors negatively affecting mothers' breastfeeding self-efficacy at a rate of 27% (F = 11.603, p < 0.001): low spouse/partner relationship satisfaction, unhealthy communication, unhealthy attention, unhealthy behaviour control, and feeding the baby only formula. Breastfeeding self-efficacy of the mothers who have high relationship satisfaction, have healthy family functions, and only breastfeed their babies is positively affected.