Background: To assess the clinical outcomes of short-term oral corticosteroid therapy for impingement syndrome of the shoulder and determine whether it can be substituted as an alternative to the intra-articular injection. Methods: The clinical outcomes of the 173 patients, the oral steroid group (n=88) and the injection group (n=85), were measured at 3 weeks, 2, 4, and 6 months postoperatively. The clinical outcomes were assessed by measuring the the University of California at Los Angeles (UCLA) score, visual analog scale (VAS) and range of motion (ROM) at every follow-up. Any complications and recurrence rate were noted. A relationship between the treatment outcomes and factors such as demographic factors, clinical symptoms and radiographic findings were determined. Results: No difference was observed in VAS and UCLA scores between the two groups, but forward flexion and internal rotation of ROM were significantly improved in the injection group at the 2nd and 4th postoperative month (p < 0.05). At 6th postoperative month, recurrence rate of symptoms was 26% in the oral steroid group and 22% in the injection group. No major adverse effects were observed. When the clinical outcomes of the oral steroid group were compared to either demographic, clinical symptoms, or radiographic findings, UCLA score was found to be significantly low (p < 0.05) in patients with joint stiffness and UCLA score, whereas VAS score was significantly improved in patients with night pain (p < 0.05). Conclusions: Short-term low-dose oral corticosteroid therapy of impingement syndrome showed comparable clinical outcomes to intra-articular injection without any remarkable adverse effects. Low-dose oral steroids can be regarded as a partial alternative to intra-articular injection for the initial therapy of impingement syndrome of the shoulder.