It is often straightforward to distinguish glioblastoma (GBM) from metastatic carcinoma by cytology; however, small cell variants of GBM or GBM with primitive neuronal component (GBMPNC) can mimic metastatic small cell carcinoma (SCC). Herein, we report a case of GBMPNC mimicking metastatic SCC and present cytological and ultrastructural findings. A 65-year-old man with memory disturbance was hospitalized. Magnetic resonance imaging revealed the presence of a 6 cm sized tumor in the right anterior temporal lobe. Intraoperative cytology slides indicated that the tumor consisted of small-sized cells with scant cytoplasm showing high cellularity. The initial intraoperative diagnosis was metastatic SCC; however, any primary visceral tumor was not detected clinically. Immunohistochemical and ultrastructural studies of postoperative histological sections revealed that the lesion was GBMPNC. This case shows that some GBMs may have the potential to closely mimic metastatic SCC, which expands the differential diagnosis and emphasizes the importance of clinical correlation.