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

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