The article discusses the problem of developing and testing a psychodiagnostic tool for measuring family social capital in a socio-psychological approach. It is noted that despite the high theoretical interest in the concept of social capital, there is currently a need for valid and reliable methods suitable for Russian families. The aim of the study is to substantiate and validate the author's methodology «Diagnosis of Family Social Capital», which reflects the structural and content characteristics of this phenomenon. In accordance with the aim, a theoretical analysis of the concept of family social capital was carried out, its key components (trust, family identity, values, norms of interaction), a set of questionnaire statements was developed, empirical testing and verification of psychometric properties – reliability, construct and convergent validity – were carried out. The empirical part of the study was conducted on a sample of 275 respondents aged 18 to 57 living in various cities in Russia. Confirmatory factor analysis methods were used to test construct validity, while Cronbach's alpha coefficient, intraclass correlation (ICC), test-retest method, and Bland-Altman analysis were used to assess reliability. Convergent validity was tested using the Trust Scale, FACES-3, a modification of Inglehart's methodology, and methods for measuring ingroup identification and intrafamilial social capital (D.I. Dubrov). The results of the data analysis showed the adequacy of the four-factor model, high indicators of internal consistency and temporal stability. Statistically significant correlations with external criteria were found, confirming the convergent validity of the instrument. It was concluded that the developed methodology «Diagnosis of Family Social Capital» is a reliable and valid diagnostic tool and can be used for research purposes.