Gaster may refer to:
- Stomach (Greek: Gaster)
- Gaster (insect anatomy)
- a trade name of famotidine, an inhibitor of stomach acid production
- W. D. Gaster, a character from the video game Undertale.
- Gaster (surname)
- Gaster, a character in the television series PaRappa the Rapper
- Gaster (district), a constituency in St. Gallen, Switzerland
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