The hematoma cavity typically remains collapsed after intracranial hematoma evacuation, but here we report an adult case in which the cavity expanded into a cystic form, developing into a porencephalic cyst, after the removal. A 73-year-old woman was admitted to our hospital presenting with a disturbance of consciousness. A CT scan showed a subcortical hemorrhage with a blend sign and brain herniation, prompting emergent hematoma evacuation. Although there was an improvement in her symptoms after the operation, her consciousness deteriorated on the 14th day post-operation. A CT scan revealed a porencephalic cyst. Reoperation was performed, and no further recurrence of the cyst was noted. Reports of hematoma cavities expanding into a cystic form after hematoma removal are rare, and the mechanisms of such cyst expansion are not well understood. In this case, we hypothesize two pathways: 1) Residual hematoma components moving through the ventricular wall due to osmotic pressure differences between the hypertonic fluid and cerebrospinal fluid (CSF), and 2) Protein-rich plasma components leaking out of cells due to blood-brain barrier (BBB) disruption caused by cerebral hemorrhage. We also consider the possibility of a check valve mechanism.