Objective: To determine the efficacy of concentric vs. eccentric exercise in improving shoulder function, pain, and tendon characteristics for patients with rheumatoid arthritis and rotator cuff tendinopathy. Methods: Forty patients with rheumatoid arthritis and rotator cuff tendinopathy were divided into either concentric or eccentric exercise groups, with 20 patients in each group. Patients received 12 sessions at a pace of 3 sessions per week. Shoulder Pain and Disability Index (SPADI), the visual analogue scale (VAS), supraspinatus and subscapularis thickness, echo pixels, and the Disease Activity Score-28-erythrocyte sedimentation rate (DAS-28-ESR) were assessed at baseline and post-treatment. Results: There was a significant difference between the concentric and eccentric groups regarding SPADI and VAS scores in favor of the eccentric group. However, there was no significant difference between the two groups regarding tendon thickness, echo pixels, or DAS-28-ESR. Conclusion Eccentric exercises were more effective than concentric exercises in improving shoulder function and pain intensity. However, neither of the two types of exercises was superior in improving tendon characteristics or disease activity.