Background/Objectives: Effective communication between patients and healthcare providers is essential for informed decision making in cancer care. Communication aids that can help prostate cancer patients optimize their involvement in treatment care planning are not widely used in the U.S. This research details the adaptation and validation process of a patient prompt list for localized prostate cancer patients undergoing treatment decisions. Methods: This process occurred in three steps: Step 1 involved gathering usable questions from the literature; in Step 2, we evaluated the tool’s content via expert (N = 6) feedback; and in Step 3, we assessed the usefulness of the prompt list with patients (N = 30). Results: Sixty percent of candidate questions (20/33) were retained for inclusion after achieving acceptable item-level content validity index (range: 0.83–1.00) and scale-level content validity index (S-CVI = 0.96) scores. The final 20 questions were classified into 5 domains (1: Treatment Options and Information, 2: Side Effects, Risks, and Recovery, 3: Emotional and Social Support, 4: Logistical and Practical Concerns, and 5: Future Health Considerations) through a card sorting exercise with a subset of patients (N = 3) and providers (N = 2). Most patients rated the prompt list as “useful or very useful” both to themselves (80%, N = 24) and to other men presenting with prostate cancer (83%, N = 25). Conclusions: The participatory process used to develop and validate the prompt list offers insights for the development of similar tools.