Background: Adolescent and young adult (AYA) cancer patients experience complex physical and psychosocial development and diverse lifestyle changes. Therefore, each may have generation-specific needs. This study aimed to develop the Japanese version of the Cancer Needs Questionnaire – Young People (CNQ-YP-J) and verify its reliability and validity among AYA cancer patients in Japan. Methods: The CNQ-YP-J was developed using a standardized translation methodology. Content validity was assessed by a group of experts, and a pilot test was conducted on six AYA cancer patients. About 87 Japanese patients with AYA cancer participated in this study. After exploratory factor analysis was conducted, the scale’s reliability was examined using Cronbach’s α, item-total correlations, and intraclass correlation coefficient (ICC) of the retest. Criterion-related validity was analyzed using correlations between total needs and concerns about physical effects and quality of life (QOL). Results: Factor analysis showed an eight-factor structure, different from the original scale, with one item excluded, resulting in a 69-item scale. Cronbach’s α coefficient and ICC were above the minimum acceptable criterion of 0.70, demonstrating high reliability. Concerning criterion-related validity, high needs were positively correlated with high concerns about physical effects and tended to be negatively correlated with QOL. Conclusion: The CNQ-YP-J is a reliable and valid scale to comprehensively assess the needs of AYA cancer patients in Japan.