Posidonia is a genus of seagrasses.
"Posidonia" may also refer to:
- Posidonia (band), an Italian band, formed in Turin
- Paestum, a Greek city in Italy, sometimes called "Posidonia" or "Poseidonia"
- Poseidonia or Posidonia, a village on the island of Syros (Greece)
- Posidonia Shale, a geological formation from the Lower Jurassic
- Posidonia an extinct genus of ostreoidean bivalve from the Paleozoic and Mesozoic