Follicular dendritic cell sarcoma (FDCS) is a rare neoplasm that is composed of cells with morphological and immunophenotypic features of follicular dendritic cells. A minority of these tumors are found on extranodal sites. We describe the case of a 58-year-old woman referred to the hospital emergency department by her general practitioner due to new-onset bicytopenia and petechial rash. The diagnostic workup showed an iron and folic acid-deficient normocytic normochromic anemia, and she was admitted to hospitalization. Further investigation revealed Helicobacter pylori infection, later eradicated, and the patient was started on oral dexamethasone over the suspicion of immune thrombocytopenic purpura. An abdominal CT scan was performed that revealed a non-contrast-enhancing 55 mm heterogeneous nodule of the spleen. A PET-CT scan with fluorodeoxyglucose (18F) was also performed, and it revealed a hypermetabolic spleen lesion that raised suspicion of a malignant neoplasm with high metabolic activity. A multidisciplinary team discussed the clinical case, and splenectomy was proposed. After appropriate vaccination, splenectomy was performed, and through macroscopical, microscopical, and immunohistochemical evaluation, the diagnosis of FDCS of the spleen was made. After the splenectomy, the patient presented with bicytopenia resolution, and at the 12-month follow-up, there was no evidence of recurrence of the neoplasm. This clinical case aims to raise awareness about this rare disease and the need for further studies to better understand this entity and its clinical evolution and to define a standard therapeutic protocol, which is currently nonexistent.