This study aimed to co-design and develop a user-centred, theory-based eHealth-mediated self-management support follow-up prototype for adults with musculoskeletal disorders. A three-step system development cycle was employed. Step 1 involved creating intervention features and content, with two focus groups reviewing prioritised eHealth intervention elements based on earlier research. Step 2 involved heuristic testing using Nielsen's 10 heuristic principles. Step 3 incorporated qualitative think-aloud interviews and the System Usability Scale. Republic of Ireland. Step 1 included adults with musculoskeletal disorders (n = 12). Step 2 involved five reviewers. Step 3 included people with musculoskeletal disorders (n = 5) and musculoskeletal physiotherapists (n = 5). Participants in step 1 approved four main intervention components, which map to recognised theoretical frameworks, and suggested increased use of visual and interactive elements. Heuristic testing in step 2 identified design and navigation issues. In Step 3, usability testing, additional navigation, content and design recommendations were identified. The overall median system usability score (interquartile range) was 75 (0) out of 100 for adults with musculoskeletal disorders and 77.5 (2.5) out of 100 for musculoskeletal physiotherapists, indicating good usability. A theory-based, user-centred eHealth-mediated follow-up self-management support prototype has been developed for people with musculoskeletal disorders, with the next steps focusing on feasibility testing in clinical practice, with a more diverse population.