Objective: Rare coexistence of disease or pathology Background: Fluid overload-associated large B-cell lymphoma (FO-LBCL) is a recently described malignant lymphoma that presents with serous effusions in the pleura, peritoneum, and/or pericardium but without an identifiable lymphoma mass. This report describes the case of an 80-year-old man who presented with a pleural effusion and describes the approach to diagnosis and management of FO-LBCL. Case Report: We present a case of an 80-year-old man who presented with right pleural effusion and shortness of breath at work. Initial radiological assessment suggested a pleural effusion on the right side, without an identifiable mass, given the patient's symptoms and imaging characteristics. Subsequently, he underwent a pleural fluid puncture and biopsy. Based on the initial pathological assessment, malignant lymphoma, a non-epithelial tumor, was considered likely, but differentiation from reactive proliferative cells was difficult, given the patient's symptoms and cytologic characteristics. Postoperatively, histopathological examination and immunohistochemistry confirmed a diagnosis of FO-LBCL. After 1 year of follow-up, the condition had progressed and the patient died due to recurrence. Conclusions: This report has presented a case of FO-LBCL in an elderly man with pleural effusion and described how this rare and recently described lymphoma was diagnosed and managed.