This report describes a case of acute pupillary block glaucoma related to migration of nonemulsified silicone oil into the anterior chamber (AC) in a young phakic patient. A 24-year-old male diabetic patient underwent uneventful left eye pars plana vitrectomy (PPV) with silicon oil endotamponade for diabetic macula-off tractional retinal detachment. Two weeks after discharge, he presented with severe left eye pain. Examination revealed hand motion vision, high intraocular pressure (IOP) of 67 mmHg with ciliary injection, corneal edema, and two large nonemulsified silicone oil bubbles in the AC at the pupillary margin. Medical management with topical antiglaucoma medications (AGMs) and intravenous acetazolamide and mannitol failed to reduce the IOP. The patient underwent left eye PPV, silicone oil removal, and AC wash. IOP was eventually controlled after the operation without AGM. Pupillary block glaucoma after silicone oil injection is well recognized in aphakic patients, but ophthalmologists should be aware that it can occur in phakic and pseudophakic patients, particularly in complicated cases and patients with a weakness of the iris–lens diaphragm.