Objective: The aim of this study was to explore the factors associated with posttraumatic growth (PTG) among adolescents bereaved owing to parental cancer in Japan. Methods: An anonymous cross-sectional Web-based survey was conducted, enrolling adolescents bereaved because of parental cancer in the previous 5 years. Posttraumatic growth, number of social support members, support from medical staff, and behaviors after bereavement were measured. Multiple linear regression was performed to explore the association between total PTG score and possible related factors. Results: We assessed 57 participants in this study. Mean age was 19.3 (standard deviation [SD] = 2.0) years, and most participants were female (75.4%). Mean total score of the Japanese version of the PTG inventory was 43.0 (SD = 25.6). Participants’ mean number of social support members was 2.1 (SD = 1.3), and these support members were the surviving parent (66.7%), friends (38.6%), and siblings (36.8%). The multiple linear regression model explained 45% of the variance in PTG. In this model, the following 3 behaviors after bereavement were associated with PTG: “putting palms together in front of a parent’s picture or an altar” (β =.36, P =.006), “visiting a parent’s grave” (β =.29, P =.03), and “having fun with friends” (β =.25, P =.04). Conclusion: Parentally bereaved adolescents in Japan experience PTG. Specific behaviors after bereavements might be recommended for PTG among Japanese adolescents bereaved because of parental cancer.