A ship prefix is a combination of letters, usually abbreviations, used in front of the name of a civilian or naval ship that has historically served numerous purposes, such as identifying the vessel's mode of propulsion, purpose, or ownership/nationality. In the modern environment, prefixes are cited inconsistently in civilian service, whereas in government service a vessel's prefix is seldom omitted due to government regulations dictating that a certain prefix be used. Today the common practice is to use a single prefix for all warships of a nation's navy, and other prefixes for auxiliaries and ships of allied services, such as coast guards. For example, the modern navy of Japan adopts the prefix "JS" – Japanese Ship, or the US navy has adopted the USS prefix. However, not all navies use prefixes. Among the blue-water navies, those of France, Brazil, China, Russia, Germany, Ukraine, and Spain do not use ship prefixes. NATO designations such as FS (French Ship), FGS (Federal German Ship), and SPS (Spanish Ship) can be used if needed.^{[citation needed]}

Historically, prefixes for civilian vessels often identified the vessel's mode of propulsion, such as "MV" (motor vessel), "SS" (screw steamer; often cited as "steam ship"), or "PS" (paddle steamer).

These days, general civilian prefixes are used inconsistently, and frequently not at all. In terms of abbreviations that may reflect a vessel's purpose or function, technology has introduced a broad variety of differently named vessels onto the world's oceans, such as "LPGC" (liquified petroleum gas carrier), or "TB" (tug-boat), or "DB" (derrick barge). In many cases though, these abbreviations are used for purely formal, legal identification and are not used colloquially or in the daily working environment. Prefixes indicating a vessel's purpose (e.g., "RMS" for a Royal Mail ship or "RV" for research vessel) are also used.^{[citation needed]}

Prefixes used for naval ships primarily reflect ownership, but may also indicate a vessel's type or purpose as a sub-set. Historically, the most significant navy was Britain's Royal Navy, which has usually used the prefix "HMS", standing for "His/Her Majesty's Ship". The Royal Navy also adopted nomenclature that reflected a vessel's type or purpose, e.g. HM Sloop. Commonwealth navies adopted a variation, with, for example, HMAS, HMCS, and HMNZS pertaining to Australia, Canada, and New Zealand, respectively.

In the early days of the United States Navy, abbreviations often included the type of vessel, for instance "USF" (United States Frigate), but this method was abandoned by President Theodore Roosevelt's Executive Order No. 549 of 1907, which made "United States Ship" (USS) the standard signifier for USN ships on active commissioned service. United States Navy prefixes officially only apply while a ship is in active commission, with only the name used before or after a period of commission and for all vessels "in service" rather than commissioned status.

However, not all navies used prefixes; this includes the significant navies of China, France and Russia.^{[citation needed]}

From the 20th century onwards, most navies identify ships by letters or hull numbers (pennant numbers) or a combination of such. These identification codes were, and still are, painted on the side of the ship. Each navy has its own system: the United States Navy uses hull classification symbols, and the Royal Navy (e.g. "D35" is destroyer 35 – HMS Dragon) and other navies of Europe and the Commonwealth use pennant numbers.^{[citation needed]}

These tables list both current and historical prefixes known to have been used.