AbstractA 9‐month‐old, male, neutered, Persian cross cat was referred for investigation of left‐sided mydriasis. Ophthalmic examination of the left eye revealed marked vitreous prolapse into the anterior chamber, which was shallow, complete mydriasis with the iris nearly contacting the cornea, lens subluxation (nasally) and an increased intraocular pressure (52 mmHg). A diagnosis of aqueous humour misdirection syndrome was made. Surgical intervention was elected due to marked intraocular changes and increased intraocular pressure. Anterior vitrectomy and lensectomy via phacoemulsification were performed. Early signs of aqueous humour misdirection syndrome developed in the right eye within 24 hours of left‐sided surgery. Medical management was initially pursued in the right eye due to a mild presentation, but surgery was subsequently required, 8 days later, when the aqueous humour misdirection syndrome progressed. This case report describes an unusual presentation of rapidly progressive bilateral aqueous humour misdirection syndrome. In such cases, early surgical intervention would be recommended to prevent secondary glaucoma and vision loss.