Background: The field of targeted radionuclide therapy (RNT) for prostate cancer (PC) is growing rapidly with recent FDA approval of the first lutetium-177-PSMA-ligand. Commonly used patient-reported outcome (PRO) measures were originally designed to assess the impacts of traditional therapies (e.g., chemotherapy, surgery). We aimed to develop the first PRO measure for PC patients receiving RNT. Methods: We used a multi-step approach based on input from experts and patients. We identified symptoms and toxicities potentially relevant in the context of RNT for PC from a literature review of published trials and interviews with PC patients who had received RNT, caregivers of RNT recipients, and clinicians. We identified items to include in the measure from the Functional Assessment of Chronic Illness Therapy (FACIT) item library. Finally, we reviewed, refined, and finalized the new measure. Results: Potentially relevant symptoms/toxicities were identified by reviewing published trials (e.g., xerostomia, nausea). Interviews with PC patients who received RNT (n=29), caregivers (n=14), and clinicians (n=11) yielded additional symptoms/toxicities (e.g., fatigue, constipation). After selecting a broad and comprehensive list of items from the FACIT item library, we refined the list based on input from 9 experts in the field of RNTs and PROs from 5 institutions. Patients who received RNT for PC (n=11) reviewed the draft measure and confirmed items were relevant to RNT recipients and comprehensible. This resulted in a brief 15-item measure deemed by key stakeholders to be relevant and useful in the context of RNT for PC. Conclusions: The FACT-RNT (Functional Assessment of Cancer Therapy – Radionuclide Therapy) is a PRO new measure for RNT recipients developed through multi-step collaboration with patients, caregivers, clinicians, and international experts. The FACT-RNT can be used in RNT clinical trials and real-world settings.