Extraorbital idiopathic pseudotumors of the skull base are very uncommon. We report the case of a 50-year-old woman who presented with left ophthalmoplegia and vision loss. Imaging studies revealed an enhancing lesion involving the left petrous apex and cavernous sinus. A transnasal endoscopic approach was used to obtain a biopsy of the left petrous apex. Pathology identified the lesion as an idiopathic pseudotumor. The patient was treated with high-dose steroids and steroid-sparing immunomodulators, and she experienced a significant improvement. To the best of our knowledge, this is the first reported case of a transnasal endoscopic approach to a biopsy of a pseudotumor involving the petrous apex. We discuss the features of this case, and we review the literature on this condition.