It contains nine species of marine plants[3] ("seagrass"), found in the seas of the Mediterranean and around the south coast of Australia.

The APG system (1998) and APG II system (2003) accept this genus as constituting the sole genus in the family Posidoniaceae, which it places in the order Alismatales, in the clade monocots.

The AP-Website[4] concludes that the three families Cymodoceaceae, Posidoniaceae and Ruppiaceae form a monophyletic group.

Posidonia oceanica has nitrogen fixation capabilities via symbiosis and other species may as well.

[6][11] In 2006 a clonal colony of P. oceanica was discovered south of the island of Ibiza which measured 8 km across and is possibly up to 100,000 years old.