We report a case of isolated unilateral complete pupil involving third cranial nerve palsy due to pituitary adenoma with parasellar extension into the right cavernous sinus. The patient was referred to us from neurosurgery with sudden onset binocular vertical diplopia with complete ptosis, and mild right-sided headache of 5-day duration. Ocular examination revealed pupil involving third cranial nerve palsy in right eye while rest of the examination including automated perimetry was normal. MRI brain with contrast revealed a mass lesion with heterogenous enhancement in the sella suggestive of a pituitary macroadenoma with possible internal haemorrhage (apoplexy). In addition, the MRI showed lateral spread to the right cavernous sinus which was causing compression of the right third cranial nerve. The patient was systemically stable. This report highlights a unique case as the lesion showed a lateral spread of pituitary adenoma without compression of the optic chiasm or other cranial nerves.