Objective Bilateral frozen shoulder (FS) is often treated with intra-articular corticosteroid injection (IACI). No studies have been performed to establish whether IACI must be administered in both shoulders or in only one shoulder to improve function. This study was therefore performed to determine whether unilateral IACI improves shoulder pain and passive range of motion (pROM) in bilateral FS. Methods The medical records of 165 patients with bilateral primary FS who underwent ultrasonography-guided IACI (2 mL of 10-mg/mL triamcinolone acetonide mixed with 5 mL of 1% lidocaine) in one shoulder were retrospectively reviewed. The outcome measures, namely the numeric rating scale (NRS) scores and pROM values (abduction, external rotation, flexion, hyperextension, and internal rotation), were evaluated pre- and post-injection. Results The patients’ mean age was 54.0 ± 8.0 years. The mean symptom duration was 6.5 ± 2.8 months. The mean follow-up period after injection was 6.7 ± 0.8 weeks. The NRS scores and pROM values significantly improved in both the injected and non-injected shoulders. Conclusions This study showed that unilateral IACI in patients with bilateral FS improves the clinical outcome of the non-injected shoulder. We suggest that physicians observe the non-injected shoulder after unilateral injection rather than performing bilateral injections.