The authors report a case of a 70-year-old man who presented with a long history of left pulsatile proptosis, irritation, and a pressure sensation behind the eye. Initial computed tomography brain demonstrated the presence of a suspected ethmoid sinus mucocele. Endoscopic sinus surgery was performed to drain the mucocele. On cautious opening of the nasal mucosa, cerebrospinal fluid was encountered requiring subsequent repair with local mucosal flap and fat graft. On further magnetic resonance imaging, the lesion was confirmed to be giant pseudomeningocele arising from the sphenoid sinus, with extensive invasion and erosion of the middle cranial fossa including the left lateral orbital wall. Here, the authors report, to the best of their knowledge, the first case of spontaneous giant pseudomeningocele in the middle cranial fossa causing pulsatile proptosis. In examining this case, the authors will also discuss the pathogenesis, diagnosis, and management of giant pseudomeningoceles, especially in the situation of a misdiagnosis.