A jetty is a long, narrow marine structure, usually made of wood, earth, stone, or concrete, that is built on a coastline and projects from the shore into the water. Its main purpose is to act as a barrier against erosion of the shoreline from currents, tides and waves from the water body.

A jetty can also be used to connect the land with deep water, further away from the shoreline, for the purpose of berthing vessels in harbours or offshore, where harbour facilities are not available. This type of jetty is called a pier.

The design of a jetty structure should take into consideration the general site and ground conditions, the civil and structural design requirements, tidal and wave phenomena, impact protection for approaching vessels, berthing and mooring, ship-to-shore transfer and access for emergency vehicles.