The capital ships of a navy are its "important" warships; the ones with the heaviest firepower and armor. There is usually no formal criterion for the classification, but it is a useful concept when thinking about strategy, for instance to compare relative naval strengths in a theater of operations without having to get bogged down in the details of tonnage and gun diameters.

In the 20th century, typical capital ships would be battleships, battlecruisers, heavy cruisers, and aircraft carriers. In the 21st century, the aircraft carrier is the last remaining capital ship, with firepower defined in decks available and aircraft per deck, rather than in tubes and calibres.

The definition of "capital ship" was formalized in the limitation treaties of the 1920s and 30s; see Washington Naval Treaty, London Naval Treaty, and Second London Naval Treaty.

Before the advent of the all-steel navy in the late 19th century, a capital ship was a warship of the first, second or third rate:

• 1st Rate: 100 or more guns, typically carried on three or four decks. Four-deckers tended to have problems with the waterline and the lowest deck seldom was able to fire except on the calmest of seas.
• 2nd Rate: 90-98 guns
• 3rd Rate: 64 to 80 guns (although 64-gun third-raters were very small and not very numerous in any era).

Frigates were ships of the fourth or fifth rate; a corvette was a ship of the sixth rate.

Fictional capital ships

In the Star Wars universe, "capital ship" refers to any starship 100 meters or longer.

In the computer game Starcraft, capital ships are considered Terran Battlecruisers or Protoss Carriers.