The saltwater marsh is an area of low ground that is subject to daily flooding by salt water. It is covered with a thick mat of grasses and other plants like sedges and rushes.

Salt marshes are common along the seacoast, inside barrier islands and beaches and in estuaries. Salt marshes often extend many miles inland and are subject to tides.

The development of salt marshes depends fundamentally on a range of otherwise harsh physical and chemical conditions becoming sufficiently benign to allow (flowering) plants to colonize intertidal mudflats. These conditions are controlled by, but not necessarily directly related to, tidal inundation and other process factors. Colonization by flowering plants is thus an indication that physical and particularly hydrodynamic forces are suitable, at least temporarily, for salt marsh growth. The seaward limit of the colonists is conventionally regarded as the lower edge of the salt marsh. However, the processes leading to salt marsh development, including biological processes, are likely to have started on the mudflats seaward of the vegetation.

Salt marshes are among the most productive ecosystems on earth. Live marsh grass is not used as a source of food, but the dead plants are a source of nourishment for bottom-dwelling scavengers such as worms, fishes, shrimps, marsh snails, and crabs. Insects are also abundant in the salt marsh. Most of these insects consume living plants, and are preyed upon by the birds and fish that inhabit the salt marsh. Fishes, crabs, and shrimps live in salt marshes where stems, leaves, and roots provide food and shelter from predators. The abundance of food and protection given by marsh plants allows the young of salt marsh inhabitants to survive to adulthood.

Many fish that inhabit the marshes move with the tide. Some marsh-dwelling fishes and shrimps, such as the mummichogs and grass shrimp, remain on the marsh surface after the tide recedes. They live in potholes and standing pools of water. Very few reptiles live in salt marsh habitats.

Many birds depend on the abundant food supply found in the salt marsh. Birds such as herons and egrets feed on fishes, shrimps, and fiddler crabs.