A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems and long green, grass-like leaves. They produce seeds and pollen and have roots and rhizomes which anchor them in seafloor sand.

Seagrasses form dense underwater meadows which are among the most productive ecosystems in the world. They provide habitats and food for a diversity of marine life comparable to that of coral reefs. This includes invertebrates like shrimp and crabs, cod and flatfish, marine mammals and birds. They provide refuges for endangered species such as seahorses, turtles, and dugongs. They function as nursery habitats for shrimps, scallops and many commercial fish species. Seagrass meadows provide coastal storm protection by the way their leaves absorb energy from waves as they hit the coast. They keep coastal waters healthy by absorbing bacteria and nutrients, and slow the speed of climate change by sequestering carbon dioxide into the sediment of the ocean floor.

Seagrasses evolved from marine algae which colonized land and became land plants, and then returned to the ocean about 100 million years ago. However, today seagrass meadows are being damaged by human activities such as pollution from land runoff, fishing boats that drag dredges or trawls across the meadows uprooting the grass, and overfishing which unbalances the ecosystem. Seagrass meadows are currently^[when?] being destroyed at a rate of about 3 square metres per second (1,900 sq ft/min).

Seagrasses are flowering plants (angiosperms) which grow in marine environments. They evolved from terrestrial plants which migrated back into the ocean about 75 to 100 million years ago. In the present day they occupy the sea bottom in shallow and sheltered coastal waters anchored in sand or mud bottoms.

There are four lineages of seagrasses  containing relatively few species (all in a single order of monocotyledon). They occupy shallow environments on all continents except Antarctica: their distribution also extends to the High Seas, such as on the Mascarene Plateau.

Seagrasses are formed by a polyphyletic group of monocotyledons (order Alismatales), which recolonised marine environments about 80 million years ago. Seagrasses are habitat-forming species because they are a source of food and shelter for a wide variety of fish and invertebrates, and they perform relevant ecosystem services.

There are about 60 species of fully marine seagrasses belonging to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the order Alismatales (in the class of monocotyledons). Seagrasses beds or meadows can be made up of either a single species (monospecific) or mixed. In temperate areas one or a few species usually dominate (like the eelgrass Zostera marina in the North Atlantic), whereas tropical beds are usually more diverse, with up to thirteen species recorded in the Philippines. Like all autotrophic plants, seagrasses photosynthesize, in the submerged photic zone. Most species undergo submarine pollination and complete their life cycle underwater.

Seagrass meadows are found in depths up to about 50 metres (160 ft), depending on water quality and light availability. These seagrass meadows are highly productive habitats that provide many ecosystem services, including protecting the coast from storms and big waves, stabilising sediment, providing safe habitats for other species and encouraging biodiversity, enhancing water quality, and sequestering carbon and nutrients.