Social support in children and adolescents is fundamental for preventing mental health problems. Differences in sources and types of social support may lead to differential effects on psychosocial outcomes; however, tools to assess these differences are lacking in Japan. The present study investigated the reliability and validity of the Japanese version of the Child and Adolescent Social Support Scale (CASSS) in 1,068 students aged 9-15 years. A confirmatory factor analysis showed robustness for the five-factor model, while the third-order factor model, which integrated the overall scale and four subdomains (types: emotional, informational, appraisal, and instrumental) within the five domains (sources: parent, teacher, classmate, close friend, and school) above, also met criteria for good model fit. The overall scale and all domains and subdomains demonstrated sufficient internal consistency (α = .76-.98). For construct validity, there were significant correlations between CASSS scores and prosocial behavior, internalizing or externalizing problems as assessed by the Strength and Difficulties Questionnaire (SDQ), and depressive symptoms assessed by the Depression Self-Rating Scale for Children (DSRS-C). The agreement rate between these correlation coefficients and the prespecified hypothesis was 89.6%. These results suggest that the Japanese version of the CASSS has adequate reliability and validity and can be used to assess perceived social support among Japanese early adolescents.