A pump-jet, hydrojet, or water jet is a marine system that creates a jet of water for propulsion.

The mechanical arrangement may be a ducted propeller with nozzle, or a centrifugal pump and nozzle.

The Italian inventor Secondo Campini showed the first functioning man-made pump-jet engine in Venice in 1931. However, he never applied for a patent, and since the device suffered from material problems resulting in a short life-span, it never became a commercial product.

The first person to achieve that was New Zealand inventor Sir William Hamilton in 1954.

Pump-jets were once limited to high-speed pleasure ships (such as jet skis and jet boats) and other small craft, but since 2000–2010 the desire for high-speed vessels has increased[citation needed] and thus the pump-jet is gaining popularity on larger craft, military vessels and ferries in particular. On these larger craft, they can be powered by diesel engines or gas turbines. Speeds of up to 40 knots can be achieved with this configuration, even with a conventional hull.[1]

Pump-jet powered ships are well known to be very maneuverable. Examples of ships using pumpjets are the fast patrol boat Dvora Mk-III craft, the Hamina class missile boats, all Virginia Class submarines, the Stena HSS High-speed Sea Service ferries, and the United States Littoral Combat Ships (LCS).

In the Ordovician geological period, the first known cephalopods swam by a natural built-in reciprocating hydrojet.