Background Knowledge about Minimal Important Differences (MIDs) is essential for the interpretation of continuous outcomes, especially patient-reported outcome measures (PROMS). Objective The aim of this study was to estimate the MID for the Western Ontario Rotator Cuff Index (WORC: score 0 (best) to 2100 (worst disability)) in adults with shoulder pain associated with partial-thickness rotator cuff tears, ‘symptomatic PTTs’, undergoing conservative treatment with physiotherapy. Design A prospectively-designed anchor-based MID analysis using data from a prospective prognostic study with a three-month follow-up conducted within an outpatient care setting in Germany. Methods The MID was estimated using data from 64 adults with atraumatic symptomatic PTTs who underwent three months of conservative treatment with physiotherapy. The anchor was a seven-point Global Perceived Change (GPC) scale. Results Based on a definition of the MID being the threshold of “being (at least slightly) improved” with a probability nearest to 0.90 (i.e. 9 of 10 patients achieving the MID), the MID for the WORC was estimated as −300 for ‘improved’ shoulder-related disability in 9 out of 10 patients (95% CI 8 out of 10 patients to everyone) undergoing three months of exercise-based physiotherapy for symptomatic PTTs. Conclusions This is the first published MID estimate for the WORC in adults with symptomatic PTTs of the rotator cuff undergoing typical treatment comprising conservative treatment with physiotherapy. The conceptual framework for interpretation facilitates its use in similar clinical contexts.