Tethys can refer to:
- Tethys (mythology), a Titaness in Greek mythology
- Tethys (moon), a natural satellite of Saturn
- Tethys Ocean, a Mesozoic-era ocean between the continents of Gondwana and Laurasia
- Prototethys Ocean (c. 550–330 Ma)
- Paleotethys Ocean (c. 445–75 Ma)
- Neotethys Ocean (c. 300–100 Ma)
- Alpine Tethys Sea (c. 175–30 Ma) or Paratethys Sea (isolated after 34 Ma); today's Black Sea, Caspian Sea, and Aral Sea are remnants of the Paratethys Sea
- Tethys (software), an SMTP server
- Tethys (LRAUV), a Long Range Autonomous Underwater Autonomous Vehicle
- Tethys (gastropod), genus of gastropods in the family Tethydidae