A healthy, 14-year-old boy presented with right hip pain and consequent fever after falling out of bed while sleeping. The patient could not walk and complained of severe pain with active and passive motion, which consisted mainly in extension and internal rotation of the right hip. Laboratory analysis of the peripheral blood identified leukocytosis and increased levels of acute phase reactants. Magnetic resonance imaging of the hip, which was performed with the expectation of right hip pathology, revealed cellulitis and abscess in the right psoas muscle and associated inflammatory changes in the adjacent presacral fat plane but showed no abnormal lesions in the adjacent pelvic bone and spine. Staphylococcus hominis was cultured from the blood. With empirical antibiotic therapy, the patient recovered fully. We report a case of primary psoas abscess confused with hip pathology in an immunocompetent child without underlying disease.