Background: Clinical trials of disease‐modifying therapies in PD require valid and responsive primary outcome measures that are relevant to patients. Objectives: The objective is to select a patient‐centered primary outcome measure for disease‐modification trials over three or more years. Methods: Experts in Parkinson's disease (PD), statistics, and health economics and patient and public involvement and engagement (PPIE) representatives reviewed and discussed potential outcome measures. A larger PPIE group provided input on their key considerations for such an endpoint. Feasibility, clinimetric properties, and relevance to patients were assessed and synthesized. Results: Although initial considerations favored the Movement Disorder Society‐sponsored revision of the Unified Parkinson's Disease Rating Scale (MDS‐UPDRS) Part III in Off, feasibility, PPIE input, and clinimetric properties supported the MDS‐UPDRS Part II. However, PPIE input also highlighted the importance of nonmotor symptoms, especially in the longer term, leading to the selection of the MDS‐UPDRS Parts I + II sum score. Conclusions: The MDS‐UPDRS Parts I + II sum score was chosen as the primary outcome for large 3‐year disease‐modification trials. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.