Topical Bimatoprost is a common and popular prostaglandin analog used as an ocular hypotensive agent in the treatment of glaucoma. Side effects include ocular hyperaemia, ocular pruritus, and periocular and iris pigmentary changes. Perioribital lipodystrophy is another well-documented outcome associated with chronic use of topical bimatoprost, which results in periorbital hallowing, upper eyelid sulcus deepening, eyelid retraction and enophthalmos. We report an unusual case of periocular muscle atrophy and weakness from unilateral topical bimatoprost use. Our patient had primary angle closure and experienced a right upper eyelid ptosis 2 months after she started to use topical bimatoprost in that eye. Clinical measurements of her eyelids clearly showed reduction in the function of her right levator muscle, suggesting that effects of topical bimatoprost may not be limited to periorbital fat. She was advised to stop topical bimatoprost and right ptosis correction surgery with levator muscle advancement was performed successfully. Ophthalmologists and patients should be aware of this potential rare side effect of topical bimatoprost, as it may be potentially disfiguring, especially with monocular use. However, its exact mechanism of action needs to be clarified further.