Coastal defence (or defense) and coastal fortification are measures taken to provide protection against military attack at or near a coastline (or other shoreline), for example, fortifications and coastal artillery. Because an invading enemy normally requires a port or harbour to sustain operations, such defences are usually concentrated around such facilities, or places where such facilities could be constructed. Coastal artillery fortifications generally followed the development of land fortifications, usually incorporating land defences; sometimes separate land defence forts were built to protect coastal forts. Through the middle 19th century, coastal forts could be bastion forts, star forts, polygonal forts, or sea forts, the first three types often with detached gun batteries called "water batteries". Coastal defence weapons throughout history were heavy naval guns or weapons based on them, often supplemented by lighter weapons. In the late 19th century separate batteries of coastal artillery replaced forts in some countries; in some areas these became widely separated geographically through the mid-20th century as weapon ranges increased. The amount of landward defence provided began to vary by country from the late 19th century; by 1900 new US forts almost totally neglected these defences. Booms were also usually part of a protected harbor's defences. In the middle 19th century underwater minefields and later controlled mines were often used, or stored in peacetime to be available in wartime. With the rise of the submarine threat at the beginning of the 20th century, anti-submarine nets were used extensively, usually added to boom defences, with major warships often being equipped with them (to allow rapid deployment once the ship was anchored or moored) through early World War I. In World War I railway artillery emerged and soon became part of coastal artillery in some countries; with railway artillery in coast defence some type of revolving mount had to be provided to allow tracking of fast-moving targets.

In littoral warfare, coastal defence counteracts naval offence, such as naval artillery, naval infantry (marines), or both.

Rather than the beach assault of modern amphibious operations, seaborne assaults of the classical and medieval age more often took the form of coastal raiders sailing up river and landing well inland of the coast. Prior to the invention of naval artillery that could sink hostile ships, the most that coastal defence could do was act as an early warning system, that could alert local naval or ground forces of the impending attack. For example, in the late Roman period the Saxon Shore was a system of forts at the mouths of navigable rivers, and watch towers along the coast of Britannia and Gaul.

Later in Anglo-Saxon Wessex, protection against Viking raiders took the form of coast watchers whose duty was to alert the local militia, the navy, which would attempt to intercept the raider's ships, or failing that, to destroy them after they had beached. Against smaller raiding forces, the threat of losing their ships, and their way home with their loot, was often enough to force them to curtail their attack. In addition there was a system of fortified towns, burghs, that were positioned at choke points along navigable rivers to prevent raiders from sailing inland.

Sea forts are completely surrounded by water – if not permanently, then at least at high tide (i.e. they are tidal islands).

Unlike most coastal fortifications, which are on the coast, sea forts are not. Instead, they are off the coast on islands, artificial islands, or are specially built structures. Some sea forts, such as Fort Denison or Fort Sumter, are within harbours in proximity to the coast, but most are at some distance off the coast. Some, such as for example Bréhon Tower or Fort Drum completely occupy small islands; others, such as Flakfortet and Pampus, are on artificial islands built up on shoals. Fort Louvois is on a built-up island, 400 meters (1,312 ft) from the shore, and connected to it by a causeway that high tide completely submerses. The most elaborate sea fort is Murud-Janjira, which is so extensive that one might truly call it a sea fortress.^{[according to whom?]}

The most recent sea forts were the Maunsell Forts, which the British built during World War II as anti-aircraft platforms. One type consisted of a concrete pontoon barge on which stood two cylindrical towers on top of which was the gun platform mounting. They were laid down in dry dock and assembled as complete units. They were then fitted out before being towed out and sunk onto their sand bank positions in 1942. The other type consisted of seven interconnected steel platforms built on stilts. Five platforms carried guns arranged in a semicircle around the sixth platform, which contained the control centre and accommodation. The seventh platform, set further out than the gun towers, was the searchlight tower.

In Colonial times the Spanish Empire diverted significant resources to fortify the Chilean coast as consequence of Dutch and English raids.