Primary psoas abscess is due largely to hematogenous or lymphatic spread under immunocompromised conditions, whereas secondary psoas abscess is due largely to direct spread from adjacent infected structures. Trauma or hematoma within the muscle may predispose to the development of a primary abscess, especially if infection is present prior to injury, despite the absence of previous signs or symptoms of infection. This report describes a 17-year-old female high school student who developed an abscess within her iliacus muscle due to methicillin-susceptible Staphylococcus aureus after running 3 km on a hill as a school exercise. She was positive for antinuclear antibody and had had atopic dermatitis, suggesting that these factors, as well as exercise-related minor trauma or hematoma within the muscle, may have predisposed to abscess formation. She was treated with appropriate antibiotics and surgical drainage, resulting in recovery after 4 weeks.