BACKGROUND Myositis is an inflammatory myopathy that can be caused by a variety of drugs, diseases, and toxins. The U.S. military uses chemoprophylaxis with intramuscular penicillin G to prevent group A streptococcal infection. We present a case of penicillin G-induced myositis, a rare cause of drug-induced myositis with limited discussion in the medical literature. CASE REPORT A 25-year-old man with no pertinent medical history presented to the Emergency Department with right hip and leg pain after receiving a single dose of intramuscular penicillin G as part of standard prophylaxis for group A streptococcal infection during basic military training. He reported pain and leg weakness that was exacerbated by physical exertion and weight bearing but had no systemic symptoms, such as fevers or chills. Initial radiographs of the hip were normal; however, subsequent magnetic resonance imaging of the hip revealed intramuscular edema and features consistent with myositis of the right proximal thigh and hip musculature. He was admitted for isolated right gluteal myositis, attributed to his preceding local penicillin injection. He recovered with symptomatic care over the following 2 weeks, with return to baseline function. CONCLUSIONS This case highlights a rare complication of intramuscular penicillin G as a cause of acute isolated myositis. It serves to inform physicians of this rare complication and to recommend the consideration of intramuscular penicillin G as a causative etiology in individuals presenting with myositis and recent penicillin G exposure.