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Retroperitoneal fibrosis (RPF) is a rare disease characterized by marked fibro-inflammatory tissue in the retroperitoneum. Approximately 70% of cases of RPF are idiopathic, while the rest can be secondary to several other causes. The diagnosis is mainly obtained by imaging modalities such as computed tomography (CT). However, histological examination should be considered when the clinical manifestations and imaging studies suggest malignancy. In particular, in malignant diseases with retroperitoneal metastases, abnormal collagen plaques are formed from an exuberant desmoplastic response, which may not be distinguishable from RPF on CT scans. Therefore, even if CT suggests RPF, biopsy is essential to identify malignant disease because it typically results in a fatal prognosis. Here, we report a case of metastatic adenocarcinoma of unknown primary site that developed only in the retroperitoneum and was initially diagnosed as RPF based on CT findings.