Ophthalmic artery pseudoaneurysm is a rare vascular lesion. Herein, we describe a rare case of an infectious pseudoaneurysm caused by a frontal sinus cyst. An 87-year-old man diagnosed as having orbital cellulitis was admitted to our hospital. Paranasal sinus CT imaging showed a frontal sinus cyst and subperiosteal abscess. Emergency endoscopic sinus surgery was performed under local anesthesia. The frontal sinus was filled with viscous pus and the dura mater was exposed. Thereafter, although the fever resolved, the patient continued to have persistent left-sided exophthalmos, and a residual subperiosteal abscess was suspected. Direct orbital puncture of the subperiosteal abscess was performed, but arterial blood was drawn instead of pus. At this point, a pseudoaneurysm was suspected and a contrast-enhanced CT confirmed ophthalmic artery pseudoaneurysm. Intravascular treatment, including arterial embolization was considered, but endovascular treatment of a pseudoaneurysm carries a high risk if the ophthalmic artery is involved. Therefore, the ophthalmic artery pseudoaneurysm was treated conservatively, and the exophthalmos gradually improved. A repeat CT conducted about 3 months after the onset confirmed disappearance of the pseudoaneurysm.