Glioblastoma (GBM) rarely presents as an infratentorial tumor in adults. The authors present a case of concomitant supratentorial and infratentorial GBM in an adult. A 72-year-old man presented with headache, nausea, vomiting, and lightheadedness. Initial MR images revealed enhancing masses in the right cerebellum and right posterior periventricular region. The patient underwent a suboccipital craniotomy and resection of the cerebellar lesion. Final histopathology was consistent with glioblastoma. The patient went on to receive standard radiation treatment for GBM with concurrent and adjuvant temozolomide. However, the patient experienced clinical deterioration within a few days after starting radiotherapy. He and his family decided to forego treatment and pursue palliative care. The patient expired three months after the initial diagnosis. Autopsy findings supported the diagnosis of GBM with leptomeningeal gliomatosis and involvement of the cerebrum, cerebellum, and spinal cord. The authors review the literature and propose that the pathogenesis of multiple and multicentric GBM may involve neural stem cells within the subventricular zone or could result from tumor dissemination along established CNS routes, such as white matter tracts and CSF pathways.