A 67-year-old man was referred to our department because of fever, right lower thigh swelling, and redness with pain. Laboratory tests showed slightly elevated leukocytes and markedly elevated C-reactive protein levels. Computed tomography confirmed a popliteal aneurysm with wall thickening, so-called mantle sign. Aneurysmorrhaphy with a reversed autologous saphenous vein reconstruction was performed. Aneurysm sac and perianeurysm tissue cultures were negative for aerobic and anaerobic bacteria. The microscopic appearance of the aneurysm showed thickening of the adventitia and infiltration of inflammatory cells. This report presents, for the first time, findings suggestive of an inflammatory aneurysm of the popliteal artery.