Basidiobolomycosis is an uncommon emerging fungal infection caused by . It frequently causes cutaneous infection, but it rarely infects visceral tissues in humans. Here, a 39-year-old previously healthy woman presented with severe left-sided abdominal pain and weight loss. She had visited several hospitals and had provisionally been diagnosed as having either a retroperitoneal malignancy or retroperitoneal fibrosis before being referred to our hospital. Abdominal computerized tomography and biopsy of the retroperitoneal mass revealed retroperitoneal basidiobolomycosis infection. She was started on antifungal treatment. This led to significant improvement, without surgical intervention. Gastrointestinal basidiobolomycosis can present in many forms, commonly involving the colon and liver with multifocal inflammatory masses. Nonetheless, retroperitoneal basidiobolomycosis presentation is extremely rare and should be considered in the differential diagnosis of a retroperitoneal mass with eosinophilia.