We report a case of posttraumatic false aneurysm of popliteal artery after a total knee arthroplasty in an 82-year-old woman. This case is characterized by the distinct history of trauma to the popliteal fossa in the immediate postoperative period and the location of false aneurysm. In addition, the clinical symptoms were similar to those of deep vein thrombosis. The ultrasonographic examination and computed tomography (CT) angiography confirmed the diagnosis on 6(th) day after the surgery. Ultrasonographic examination 1 day later revealed no increase in the size of false aneurysm. She was treated with open surgical repair. Presenting the difficulty in the diagnosis, unusual location, etiology, and its management is the intention of this case report.