We report a rare case of a pseudoaneurysm of the posterior circumflex humeral artery, probably related to a steroid injection in the shoulder 3 months earlier. The clinical presentation was atypical and the location not previously described in the literature. The patient was referred with a painless, nonpulsatile mass in the posterior shoulder region and was suspected of a malignant soft-tissue tumor. Sonography, including power Doppler imaging, demonstrated a pseudoaneurysm, with the intralesional blood-filled cavity developed from the posterior circumflex humeral artery. The diagnosis was confirmed by pathologic examination of the surgical specimen.