Background: Chronic shoulder tendinopathy, particularly of the rotator cuff, causes persistent pain, functional limitation, and reduced quality of life. Conservative treatments often provide limited relief, while corticosteroid injections offer short-term benefits but may weaken tendons and lead to symptom recurrence. Platelet-rich plasma (PRP), an autologous concentration of growth factors, promotes collagen synthesis, angiogenesis, and tendon healing, showing superior short-term pain and functional improvement in previous studies. Methods: In this prospective, randomized controlled trial, 60 patients with chronic shoulder tendinopathy were assigned to receive ultrasound-guided PRP (n = 30) or corticosteroid (n = 30) injections. Outcomes pain (VAS), shoulder function (ASES), range of motion, tendon healing on ultrasound, and adverse events were assessed at baseline, 6 weeks, 3 months, and 6 months. Data were analyzed using t-tests, chi-square tests, and ANOVA; p < 0.05 was considered significant. Results: Baseline characteristics were similar between groups. Both treatments improved pain and function, but PRP provided significantly greater pain relief at 3 months (VAS 2.3 vs 3.5; p = 0.01) and 6 months (1.9 vs 3.1; p < 0.001), and higher functional scores at 3 months (ASES 78.6 vs 70.3; p = 0.002) and 6 months (85.9 vs 75.2; p < 0.001). PRP also resulted in superior shoulder abduction (48° vs 34°; p = 0.01) and external rotation (19° vs 11°; p = 0.02), and higher rates of tendon healing on ultrasound (echotexture 63.3% vs 33.3%; p = 0.03; hypo echogenicity 56.7% vs 30.0%; p = 0.04). Both interventions were safe, with only mild transient pain or swelling and no serious complications. Conclusion: Ultrasound-guided PRP injections provide superior mid-term pain relief, functional improvement, shoulder mobility, and tendon healing compared to corticosteroids in chronic shoulder tendinopathy, with comparable safety, supporting PRP as a regenerative alternative for long-term management.