To report a case of ocular hypertension due to swelling and degeneration of hydrogel explant (MIRAgel) after retinal detachment surgery. The patient who had a history of left retinal detachment 23 years prior had been regularly followed up for epiretinal membrane in the left eye at the Department of Ophthalmology, Hiroshima University Hospital. Two years after the first presentation, the patient had symptoms of foreign body sensation and hyperemia, with elevation of the intraocular pressure (IOP) of the left eye to 24 mmHg. Two months later, the patient noticed omnidirectional oculomotor disturbances in the left eye, and magnetic resonance imaging (MRI) revealed swelling of the buckle material, presumably hydrogel explant, surrounding his left eye. His oculomotor disturbances worsened, and the left eye IOP remained high at 40 mmHg, despite the administration of antihypertensive eye drops. Subsequently, the swollen hydrogel explant was surgically removed. After the surgery, there was improvement of the diplopia and foreign body sensation. However, IOP in the left eye remained at 34 mmHg, and a trabeculectomy was performed to normalize the IOP. As far as we know, there have been no reported cases of irreversible ocular hypertension due to hydrogel explant. Stenosis of the trabecular outflow pathway secondary to compression of the superior scleral vein by long-term swollen hydrogel explant and inflammation around the hydrogel explant may be the cause of irreversible IOP elevation. Trabeculectomy may be effective for treating the intraocular hypertension caused by hydrogel explant.