Background Cancer is a highly distressing and traumatic experience that can significantly impact individuals’ psychological well-being. Whereas many cancer patients may experience negative psychological outcomes, such as posttraumatic stress symptoms, a subset of individuals also reports positive psychological changes known as posttraumatic growth. Objective To contribute to theory-driven development of these treatments by exploring the role of deliberate rumination in posttraumatic growth, along with the basic beliefs that might shape it. Methods A sample of cancer patients (n = 139) completed self-report measures assessing rumination, basic beliefs, and posttraumatic growth. Multiple linear regression analyses and moderated mediation models were conducted to analyze the data. Results Findings support the hypothesis that intrusive rumination, deliberate ruminating, and disruption in basic beliefs strongly predict posttraumatic growth in cancer survivors. The results of this simple mediation indicate that deliberate rumination has a full mediating role. The results show that at all levels of basic beliefs, there is a positive and statistically significant indirect effect of intrusive rumination on posttraumatic growth. Conclusions These findings highlight the importance of addressing rumination and core beliefs in interventions aimed at promoting posttraumatic growth among cancer patients. Further research is needed to explore additional factors that may influence the relationship between rumination, basic beliefs, and posttraumatic growth. Implications for Practice Oncology nurses can assist individuals in recognizing and managing intrusive rumination, as well as advocating deliberate rumination as a constructive cognitive process for making sense of a traumatic event and fostering growth.