A breakwater is a permanent structure constructed at a coastal area to protect against tides, currents, waves, and storm surges. Breakwaters have been built since antiquity to protect anchorages, helping isolate vessels from marine hazards such as wind-driven waves. A breakwater, also known in some contexts as a jetty or a mole, may be connected to land or freestanding, and may contain a walkway or road for vehicle access.

Part of a coastal management system, breakwaters are installed parallel to the shore to minimize erosion. On beaches where longshore drift threatens the erosion of beach material, smaller structures on the beach may be installed, usually perpendicular to the water's edge. Their action on waves and current is intended to slow the longshore drift and discourage mobilisation of beach material. In this usage they are more usually referred to as groynes.

Breakwaters reduce the intensity of wave action in inshore waters and thereby provide safe harbourage. Breakwaters may also be small structures designed to protect a gently sloping beach to reduce coastal erosion; they are placed 100–300 feet (30–90 m) offshore in relatively shallow water.

An anchorage is only safe if ships anchored there are protected from the force of powerful waves by some large structure which they can shelter behind. Natural harbours are formed by such barriers as headlands or reefs. Some natural harbours, such as those in Plymouth Sound, Portland Harbour, and Cherbourg, have been enhanced or extended by breakwaters made of rock. The Mulberry harbours were mobile breakwaters that were towed into place to create an artificial harbour during the 1944 Allied invasion of Normandy.

Types of breakwaters include vertical wall breakwater, mound breakwater and mound with superstructure or composite breakwater.

A breakwater structure is designed to absorb the energy of the waves that hit it, either by using mass (e.g. with caissons), or by using a revetment slope (e.g. with rock or concrete armour units).

In coastal engineering, a revetment is a land-backed structure whilst a breakwater is a sea-backed structure (i.e. water on both sides).

Rubble mound breakwaters use structural voids to dissipate the wave energy. Rubble mound breakwaters consist of piles of stones more or less sorted according to their unit weight: smaller stones for the core and larger stones as an armour layer protecting the core from wave attack. Rock or concrete armour units on the outside of the structure absorb most of the energy, while gravels or sands prevent the wave energy's continuing through the breakwater core. The slopes of the revetment are typically between 1:1 and 1:2, depending upon the materials used. In shallow water, revetment breakwaters are usually relatively inexpensive. As water depth increases, the material requirements—and hence costs—increase significantly.