A spy ship or reconnaissance vessel is a dedicated ship intended to gather intelligence, usually by means of sophisticated electronic eavesdropping. In a wider sense, any ship intended to gather information could be considered a spy ship.

Spy ships are usually controlled by a nation's government, due to the high costs and advanced equipment required. They tend to be parts of the nation's navy, though they may also be operated by secret services.

Naval trawlers masquerade as civilian ships such as fishing trawlers, which could be reasonably expected to remain in a certain area for a long time.

Ships which are used to infiltrate spies or special forces are sometimes also called "spy ships".

An early version of what would become known as a spy ship is the United States civilian cargo ship USS Gold Star (AK-12), which made frequent voyages to Japan, China and the Philippines with cargo and passengers during the 1920s and 1930s. Starting in 1933 as a station ship she was assigned to monitor internal Japanese Fleet frequencies and direction finder azimuths. She had three intercept operators and one chief radioman supervised by an officer. Gold Star and ground stations provided significant intelligence before the Japanese attack on Pearl Harbor on 7 December 1941.

Spy ships in the modern sense of being specially built and entirely dedicated to intelligence tasks came into being during the early Cold War, and they are in use by all major powers. Their uses, in addition to listening in on communications and spy on enemy fleet movements, were to monitor nuclear tests and missile launches (especially of potential ICBMs). One of the most important functions for both Cold War spy ship fleets, especially in the 1960s, was the gathering of submarine "signatures" – the patterns of noise that could often identify the specific type of submarine and were thus valuable in anti-submarine warfare. During that era, the United States fielded about 80 vessels, usually classified as "environmental research" craft, while the Soviet Union had around 60 ships, often converted trawlers or hydrographic research ships.

In the late 1980s, the Soviet fisheries fleet was known for having equipped many of their thousands of ships with sophisticated SIGINT and ELINT equipment, thus functioning as auxiliary spy ships tracking western naval vessels and electronic communications (though their main function remained commercial fishing).

A spy ship usually stays in international waters (or at least outside territorial waters), so as to not violate territorial borders. From there, it will use its electronic equipment to monitor sea and air traffic, radio and radar frequencies and also try to intercept and decrypt coded radio or phone communications. This is mostly done via passive means such as radio receivers or passive sonar. Sometimes however, active measures such as radar or sonar may also be used to detect the movement of aircraft, missiles, ships or other vehicles or troops. However, this risks revealing the ship's purpose.^{[citation needed]}