BACKGROUND: For patients with childhood-onset chronic illnesses, the transition to adult care requires an understanding of transition readiness and the effectiveness of evaluation methods. However, no such psychometrically verified scales exist in Japan. This study aimed to develop a Japanese version of the Transition Readiness Assessment Questionnaire (TRAQ) and verify its validity and reliability. METHODS: The Japanese TRAQ was developed in accordance with international guidelines, followed by a preliminary survey to verify face validity among six participants who fulfilled the inclusion criteria. For the main survey 107 patients who fulfilled the same inclusion criteria were asked to complete the questionnaire and provide basic information. After descriptive statistics analysis, the construct validity of the Japanese TRAQ was tested using the t-test and Pearson's correlation coefficients. Cronbach's alpha coefficients were calculated to assess reliability. RESULTS: In the main survey, 76 participants with no missing data were included in the complete data analysis (40 males 36 females; mean age, 17.8 and 18.2 years, respectively). The mean total Japanese TRAQ score was 3.9. Cronbach's alpha coefficients were 0.94 overall and 0.8-0.96 for each of the four domains. The known-groups analysis revealed that older participants (r = 0.23, P = 0.044), those having knowledge of the disease name (yes [4.0] vs no [3.4]; P < 0.001), and making unaccompanied hospital visits (with parent/others [3.7] vs alone [4.4]; P < 0.001) had significantly higher total TRAQ scores. CONCLUSION: We confirmed preliminarily the validity and reliability of the Japanese TRAQ.