Posidonia is a genus of flowering plants. It comprises nine species of marine plants, located in the seas of this Mediterranean and around the south shore of Australia.

The APG system (1998) and APG II system (2003) accept this genus as advocating the only genus in the family Posidoniaceae, which it puts in the purchase Alismatales, in the clade monocots. The AP-Website[4] concludes that the three households Cymodoceaceae, Posidoniaceae and Ruppiaceae form a monophyletic group. Formerly systems categorized this genus in the family Potamogetonaceae or at the household Posidoniaceae but belonging to purchase Zosterales.