Coastal defence ships (sometimes called coastal battleships or coast defence ships) were warships built for the purpose of coastal defence, mostly during the period from 1860 to 1920. They were small, often cruiser-sized warships that sacrificed speed and range for armour and armament. They were usually attractive to nations that either could not afford full-sized battleships or could be satisfied by specially designed shallow-draft vessels capable of littoral operations close to their own shores. The Nordic countries and Thailand found them particularly appropriate for their island-dotted coastal waters. Some vessels had limited blue-water capabilities; others operated in rivers.

The coastal defence ships differed from earlier monitors by having a higher freeboard and usually possessing both higher speed and a secondary armament; some examples also mounted casemated guns (monitors' guns were almost always in turrets). They varied in size from around 1,500 tons to 8,000 tons.

Their construction and appearance was often that of miniaturized pre-dreadnought battleships. As such, they carried heavier armour than cruisers or gunboats of equivalent size, were typically equipped with a main armament of two or four heavy and several lighter guns in turrets or casemates, and could steam at a higher speed than most monitors. In service they were mainly used as movable coastal artillery rather than instruments of sea control or fleet engagements like the battleships operated by blue-water navies. Few of these ships saw combat in the First World War, though some did in the Second World War. The last were scrapped in the 1970s.

Navies with coastal defence ships serving as their main capital ships included those of Denmark, Ecuador, Finland, the Netherlands, Norway, Portugal, Sweden, Thailand, and the British colonies of India and Victoria. Some nations which at one time or another built, bought, or otherwise acquired their own front-line capital ships, such as Argentina, Austria-Hungary, Brazil, China, Germany, Russia, and Spain, also deployed this type of warship, with Russia using three at the Battle of Tsushima in 1905.

Apart from specially built coastal defence ships, some navies used various obsolescent ships in this role. The Royal Navy deployed four Majestic-class battleships as guardships in the Humber at the start of the First World War. Similarly, the U.S. Navy redesignated the Indiana and Iowa classes as "Coast Defense Battleships" in 1919. Such ships tended to be near the end of their service lives and while generally considered no longer fit for front-line service, they were still powerful enough for defensive duties in reserve situations.

This type of vessel has always been categorized differently by different countries, due to treaties, differences in judgments related to design or intended roles, and also national pride. In the United Kingdom the Scandinavian ships were known as "coast defence ships". The Germans called these ships Küstenpanzerschiff ("coastal armoured ship"). The Danes referred to their ships as Kystforsvarsskib ("coast defence ship") and Panserskib ("armoured ship"). In Norway they were referred to as panserskip ("armoured ship"). The Dutch called their ships Kruiser ("cruiser"), Pantserschip ("armoured ship") or Slagschip ("battleship"). The Swedish term for these ships was initially 1:a klass Pansarbåt ("1st class armoured boat") and later Pansarskepp ("armoured ship"). Note however, that the German Panzerschiffe of the Deutschland class were not designed as coastal defense ships but as high seas raiders.

As an example of the profusion of terms and classifications which often contradicted each other, the 1938 edition of Jane's Fighting Ships lists the Swedish Pansarskepp of the Sverige class as battleships.

The Swedish Pansarskepp were an outgrowth of the earlier Swedish adoption of the monitor and were used for similar duties.