We report a rare case of an idiopathic pseudoaneurysm causing intraventricular hemorrhage (IVH). A 28-year-old man presented with sudden onset of severe headache. He underwent external ventricular drainage for an isolated IVH in the right lateral ventricle. Digital subtraction angiography (DSA) revealed that the aneurysm (7.5x4.5 mm) arose from the distal part of the medial lenticulostriate artery. Following removal of the external ventricular drainage catheter, the aneurysm decreased in size (4.0x2.3 mm). However, follow-up DSA revealed a slightly enlarged aneurysm (4.2x3.2 mm) with morphologic change. The aneurysm was clipped via the interhemispheric transcallosal approach, but postoperative DSA revealed a residual aneurysm sac beside the clips. Given the risk of rebleeding, a second operation was planned for complete resection of the aneurysm. After revised craniotomy and careful dissection of the caudate nucleus, the aneurysm sac was completely resected. Histopathological examination revealed that the aneurysm was a pseudoaneurysm. The patient recovered without any neurological sequel and was discharged. To the best of our knowledge, this is the first reported case of an idiopathic lenticulostriate artery pseudoaneurysm protruding into the right lateral ventricle and causing an IVH that was successfully treated with microsurgical resection.