A decoy (derived from the Dutch de kooi, literally "the cage" or possibly eenden kooi, "duck cage") is usually a person, device, or event which resembles what an individual or a group might be looking for, but it is only meant to lure them. Decoys have been used for centuries most notably in game hunting, but also in wartime and in the committing or resolving of crimes.

In hunting wildfowl, the term decoy may refer to two distinct devices. One, the duck decoy (structure), is a long cone-shaped wickerwork tunnel installed on a small pond to catch wild ducks. After the ducks settled on the pond, a small, trained dog would herd the birds into the tunnel. The catch was formerly sent to market for food, but now these are used only by ornithologists to catch ducks to be ringed and released. The word decoy, also originally found in English as "coy", derives from the Dutch de Kooi (the cage) and dates back to the early 17th century, when this type of duck trap was introduced to England from the Netherlands. As "decoy" came more commonly to signify a person or a device than a pond with a cage-trap, the latter acquired the retronym decoy pool.

The other form, a duck decoy (model), otherwise known as a 'decoy duck', 'hunting decoy' or 'wildfowl decoy', is a life-size model of the creature. The hunter places a number about the hunting area as they will encourage wild birds to land nearby, hopefully within the range of the concealed hunter. Originally carved from wood, they are now typically made from plastic.

Wildfowl decoys (primarily ducks, geese, shorebirds, and crows, but including some other species) are considered a form of folk art. Collecting decoys has become a significant hobby both for folk art collectors and hunters. The world record was set in September 2007 when a pintail drake and Canada goose, both by A. Elmer Crowell, sold for 1.13 million dollars apiece.

The decoy in war is a low-cost device intended to represent a real item of military equipment.

They may be used in different ways:

In irregular warfare, improvised explosive devices (IEDs) are commonly used as roadside bombs to target military patrols. Some guerrillas also use imitation IEDs to intimidate civilians, to waste bomb disposal resources, or to set up an ambush. Some terrorist groups use fake bombs during a hostage siege, in order to limit hostage rescue efforts.

A sonar decoy is a device designed to create a misleading reading on sonar, such as the appearance of a false target.