A seawall is a structure which is designed to protect a shoreline from flooding and erosion. Seawalls are only one part of a larger coastal defense system which is designed with several protections in case one or more fail. Many visitors to sea-side towns are familiar with the concept of a sea wall, as are residents of these regions. There are a number of different ways to construct a seawall, depending on available materials and the needs which the seawall needs to meet.

Essentially, a seawall acts as a layer between the vulnerable coastline and the ocean. Wave action can beat at the seawall without eroding the coast, although the seawall itself will eventually break down and require repair or replacement. Seawalls also help to insulate communities from flooding, although high waves can still breach most seawalls. A seawall can also provide a space for recreation, since the top of a seawall is often flat, allowing people to walk on it or to fish from it.

In some cases, a seawall will be constructed on shore to break high waves which might otherwise damage structures and roads on the shore. In these instances, several lower barriers may precede the seawall, to help break up the wave energy before the waves hit the wall. Other sea walls are built in the water right next to a shoreline, as is the case on many islands. Some nations also build seawalls in the open water to act as flood barriers and to dissipate waves before they reach the shore, encouraging them to break more gently.

Many seawalls are curved, allowing waves to break against them while reducing their energy. Others are straight, and they are designed to bring waves to a standstill. It is not uncommon to bury low mounds of rubble in front of a seawall to assist with the goal of reducing the power of waves as they hit the shore. In the case of a seawall which is meant to resist flooding, the seawall may be quite high so that it can cope with storm surge.

Traditional seawalls are built from large rocks, pebbles, and other rubble; some remains of ancient seawalls built in this style can be seen in many communities. A modern seawall can be made from these materials, although it may also integrate concrete, metal bars, and other tools to make the seawall more sturdy. Cheaper seawalls may be made from wood and plastic, which provide some protection although they can potentially fail in heavy weather.