A retroperitoneal cystic mass compressing the right psoas muscle was found incidentally by ultrasonography in a 67-year-old woman. The radiological findings and a history of costal caries led us to suspect a psoas cold abscess. Ultrasound-guided needle aspiration was done to establish the diagnosis and to drain the content, but only a small amount of sterile fluid was obtained. The patient complained of neuralgia in her right leg at the time of puncture. Under the preoperative diagnosis of a neurogenic tumor, the mass was surgically resected, and found to be filled with old blood. The solid region consisted of a proliferation of fusiform cells, leading to a diagnosis of benign schwannoma. Retroperitoneal schwannoma is often misdiagnosed as an adjacent anatomical structure. Thus, we conclude that both microbiological and cytological examination of an aspiration specimen is important when psoas abscess is considered in a differential diagnosis.