Seagrass (or sea-grass in British English) are flowering plants from four plant families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae) that grow in the marine saline environment.

They are called seagrasses only because most species superficially resemble terrestrial grasses of the Family Poaceae. Because these plants must photosynthesize, they are limited to growing submerged in the photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. They undergo pollination while submerged and complete their entire life cycle underwater. There are about 60 species worldwide (although the taxonomy is still disputed).