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