Background Rotator cuff syndrome, including tendinopathy and partial-thickness tears, is a common cause of shoulder pain. While conservative management remains the first-line treatment, subacromial hyaluronic acid (HA) injections have been proposed as an alternative for patients with persistent symptoms. Methods This retrospective case series evaluated the clinical outcomes of subacromial HA injections in adults diagnosed with rotator cuff syndrome, presenting with tendinopathy or partial-thickness tears on MRI. Patients received two HA injections, two weeks apart. The primary outcome was the improvement in the American Shoulder and Elbow Surgeons (ASES) score. Secondary outcomes included the Single Assessment Numeric Evaluation (SANE) score and pain reduction using the visual analog scale (VAS). Assessments were conducted at baseline and at one, three, six, and 12 months post-injection. Results A total of 41 patients (68 shoulders) were included, with a mean age of 50.1 ± 14.5 years; 53.6% were female. The ASES score improved from 45.5 ± 17.2 at baseline to 63.4 ± 25.1 at 12 months (p < 0.001). The SANE score increased from 61% ± 17 to 76% ± 19 (p = 0.013), while VAS decreased from 6.3 ± 1.9 to 4.3 ± 3.0 (p < 0.001). There was no difference in the ASES, SANE, and VAS scores between the tendinopathy and partial tear groups at any moment. Conclusion Subacromial HA injections resulted in clinically and statistically significant improvement in ASES, SANE, and VAS scores at 12 months. The treatment provided consistent symptom relief and functional improvement over time, with no reports of complications.