We report a rare case of a 69-year-old woman in whom diffuse large B-cell lymphoma (DLBCL) originated from the uterus and involved the urinary bladder. The cervical smears of the case mostly consisted of discohesive atypical round cells, which were highly suggestive of lymphoma; however, in voided urine smears, a majority of the cells formed large aggregates of degenerated cells, mimicking those of urothelial carcinoma (UC). The smears also represented some small loose clusters, in which tumor cells formed short chains with nuclear molding, mimicking those of small cell carcinoma. The cytodiagnosis got definitive when we identified the atypical cells that showed CD20+/CD3-/cytokeratin-/NSE- immunophenotype. These are of particular concern as they may have misleading similarities to other epithelial neoplasms when examining lymphoma involving the urinary bladder. Accordingly, this case highlights the importance of immunocytochemistry to rule out malignant lymphoma when encountering large and/or small loose clusters of atypical round cells on urinary cytology.