A cruiser is a type of warship. Modern cruisers are generally the largest ships in a fleet after aircraft carriers and amphibious assault ships, and can usually perform several roles.

The term "cruiser", which has been in use for several hundred years, has changed its meaning over time. During the Age of Sail, the term cruising referred to certain kinds of missions—independent scouting, commerce protection, or raiding—fulfilled by frigates or sloops-of-war, which functioned as the cruising warships of a fleet.

In the middle of the 19th century, cruiser came to be a classification of the ships intended for cruising distant waters, for commerce raiding, and for scouting for the battle fleet. Cruisers came in a wide variety of sizes, from the medium-sized protected cruiser to large armored cruisers that were nearly as big (although not as powerful or as well-armored) as a pre-dreadnought battleship. With the advent of the dreadnought battleship before World War I, the armored cruiser evolved into a vessel of similar scale known as the battlecruiser. The very large battlecruisers of the World War I era that succeeded armored cruisers were now classified, along with dreadnought battleships, as capital ships.

By the early 20th century, after World War I, the direct successors to protected cruisers could be placed on a consistent scale of warship size, smaller than a battleship but larger than a destroyer. In 1922, the Washington Naval Treaty placed a formal limit on these cruisers, which were defined as warships of up to 10,000 tons displacement carrying guns no larger than 8 inches in calibre; whilst the 1930 London Naval Treaty created a divide of two cruiser types, heavy cruisers having 6.1 inches to 8 inch guns, while those with guns of 6.1 inches or less were light cruisers. Each type were limited in total and individual tonnage which shaped cruiser design until the collapse of the treaty system just prior to the start of World War II. Some variations on the Treaty cruiser design included the German Deutschland-class "pocket battleships", which had heavier armament at the expense of speed compared to standard heavy cruisers, and the American Alaska class, which was a scaled-up heavy cruiser design designated as a "cruiser-killer".

In the later 20th century, the obsolescence of the battleship left the cruiser as the largest and most powerful surface combatant ships (aircraft carriers not being considered surface combatants, as their attack capability comes from their air wings rather than on-board weapons). The role of the cruiser varied according to ship and navy, often including air defense and shore bombardment. During the Cold War the Soviet Navy's cruisers had heavy anti-ship missile armament designed to sink NATO carrier task-forces via saturation attack. The U.S. Navy built guided-missile cruisers upon destroyer-style hulls (some called "destroyer leaders" or "frigates" prior to the 1975 reclassification) primarily designed to provide air defense while often adding anti-submarine capabilities, being larger and having longer-range surface-to-air missiles (SAMs) than early Charles F. Adams guided-missile destroyers tasked with the short-range air defense role. By the end of the Cold War the line between cruisers and destroyers had blurred, with the Ticonderoga-class cruiser using the hull of the Spruance-class destroyer but receiving the cruiser designation due to their enhanced mission and combat systems.

As of 2020, only two countries operate vessels formally classed as cruisers: the United States and Russia, and in both cases the vessels are primarily armed with guided missiles. BAP Almirante Grau was the last gun cruiser in service, serving with the Peruvian Navy until 2017.

Nevertheless, other classes in addition to the above may be considered cruisers due to differing classification systems. The US/NATO system includes the Type 055 from China and the Slava from Russia. International Institute for Strategic Studies' "The Military Balance" defines a cruiser as a surface combatant displacing at least 9750 tonnes; it includes the Type 055, the Sejong the Great from South Korea, the Atago from Japan, the Slava, the Kidd operated by Taiwan, and the Zumwalt, Ticonderoga and Flight III Arleigh Burke from the US.