Background <p>The QUAlity of Life Assessment in Spina bifida (QUALAS) is a self-administered questionnaire that measures health-related quality of life. The Japanese versions of QUALAS for children and teenagers with spina bifida (SB) have been validated. This study aimed to develop and validate the reliability and validity of the Japanese version of QUALAS-A (QUALAS-A-J), the adult version of the instrument.</p> Methodology <p>The participants were adults with SB aged ≥ 18 years. The results of cognitive interviews and a preliminary survey conducted on 16 participants were analyzed to confirm the face and content validity of the responses, and the item wording was modified. The revised questionnaire was administered from April to December 2022. The survey requested responses regarding demographics, QUALAS-A-J, and the World Health Organization Quality of Life Scale (WHOQOL-26). We then calculated descriptive statistics and correlation coefficients, and conducted exploratory factor analysis and Student’s <i>t</i>-test. Cronbach’s α and retests were used to determine reliability and intraclass correlation coefficients (ICCs), respectively.</p> Results <p>Valid responses were received from 133 participants (52% female; mean age, 31.3 ± 10.5 years). Factor analysis indicated a 12-item, three-factor structure. Three items related to sexual activity that had low variance estimates were eliminated. Two factors converged on the same items as the original version; the correlation coefficients for QUALAS-A-J and WHOQOL-26 domains were 0.36 ≤ <i>r</i> ≤ 0.72, which confirmed discriminability for two domains. In all three domains, health-related quality of life was higher for those without than for those with urinary incontinence, validating known-groups validity. Cronbach’s α was 0.66–0.88 and the ICCs were &gt; 0.8, thereby confirming reliability.</p> Conclusions <p>The present study evaluated the reliability of QUALAS-A-J, which has three domains, 12 items, and two original and one new structure.</p>