BackgroundThe Workplace Social Capital (WSC) Scale is the most frequently used tool for measuring social capital at work in Western countries. However, there are no corresponding tools for assessing WSC among medical trainees in Japan. Thus, this study was conducted to develop the Japanese medical resident version of the WSC (JMR-WSC) Scale and examine its validity and reliability.MethodsThe Japanese version of the WSC Scale by Odagiri et al. was reviewed and the scale was partially modified for use in the Japanese context of postgraduate medical education. To verify the validity and reliability of the JMR-WSC Scale, a cross-sectional survey was performed in 32 hospitals across Japan. Postgraduate trainees (years 1–6) at the participating hospitals responded to the online questionnaire on a voluntary basis. We tested the structural validity through confirmatory factor analysis. We also examined criterion-related validity and internal consistency reliability of the JMR-WSC Scale.ResultsIn all, 289 trainees completed the questionnaire. The results of confirmatory factor analysis supported the JMR-WSC Scale’s structural validity on the same two-factor model as that of the original WSC Scale. Logistic regression analysis showed that, after adjustment for gender and postgraduate years, trainees with good self-rated health had a significantly elevated odds ratio for good WSC. Cronbach’s alpha coefficients showed acceptable internal consistency reliability.ConclusionsWe successfully developed the JMR-WSC Scale and examined its validity and reliability. Our scale could be used to measure social capital in postgraduate medical training settings in Japan to help prevent burnout and reduce patient safety incidents.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12909-023-04469-w.