Background: Promoting breastfeeding support by public health nurses (PHN) requires first that the support which they currently provide to be assessed. However, there is no assessment tool for this purpose. The aim of this study was therefore to develop a scale to assess breastfeeding support currently provided by PHN. Methods: We developed the Practice of Breastfeeding Support Scale (PBSS) for PHN based on the results of a previous study. The content validity of the PBSS was established through discussion with three other researchers. A pilot study was conducted to confirm face validity. To confirm reliability and validity, an anonymous, self-reported questionnaire was sent to PHN working in municipal offices. The statistical analyses included the Kaiser-Meyer-Olkin (KMO), Barlett’s Test of Sphericity, exploratory factor analysis (EFA), Cronbach’s alpha and correlation coefficient. Results: 768 PHN participated in this study. Cronbach’s alpha of PBSS was 0.85. The KMO measure was 0.892, and Bartlett’s Test of Sphericity was p < 0.01. Three factors together accounted for 59.3% of the variance in EFA. Construct validity was confirmed through comparison with categories from a previous study. The correlation coefficient of PBSS and Self-efficacy of Breastfeeding Support Scale were r = 0.56 (p < 0.01). PBSS comprised 15 questions and three factors including “Collecting information and assessment,” “Direct and individual support,” and “Support for group and community.” Conclusion: The reliability and validity of PBSS were confirmed. These findings suggested that the PBSS has the potential to help promote breastfeeding support by PHN by clarifying their current breastfeeding support practices and related factors.