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A gemistocyte (/ɛˈmɪstəst/ jem-ISS-tə-syte; from Greek γέμιζω (gemizo), meaning 'to fill up') is a swollen, reactive astrocyte. These cells usually appear during acute injury; after that, they gradually shrink in size.

Astrocytes participating in gliosis are referred to as reactive astrocytes. They have a large cytoplasmic mass, long, branching processes, and increased cytoplasmic filaments. Such astrocytes are also known as gemistocytic astrocytes. Gemistocytes are also found in some chronic diseases and within certain brain tumors, which suggests the presence of a long-lasting pathological reaction. In the context of cancer (gemistocytic astrocytomas), gemistocytes are known to dedifferentiate to a high grade (III or IV) glioma (i.e. glioblastoma multiforme) at a rapid pace, usually indicative of a poor prognosis.