Family wellbeing is studied worldwide. However, there is a dearth of studies on the wellbeing of families in Chinese societies such as Hong Kong, nor is there any socially relevant and culturally unique instrument for measuring such wellbeing. This paper reports the validity and reliability of an instrument that was developed to assess the wellbeing of Hong Kong Chinese families. Through a comprehensive literature review, interviews with service users from a diversity of backgrounds, consultation meetings with experts on families and related issues, and a pilot study, the framework of a family wellbeing index was developed. Using data from 1343 participants collected via a cross-sectional telephone survey, we employed exploratory factor analysis and confirmatory factor analysis to develop the factorial structure of the tool. Six domains, six subdomains, 23 single-question indicators, and 26 questions were constructed. The six domains were family solidarity, family resources, family health, social involvement, social resources, and work-life balance. The internal consistency of the overall index was 0.904. The scale had a significant predictive effect on the subjective appraisal of the current and future status of the wellbeing of a family, which provided initial evidence of convergent validity. The results of the confirmatory factor analysis confirmed that the construct of the tool encompassed different domains. This study is significant because it provides directions for future studies on family wellbeing and insights for social policy formulation and social service development.