A warning sign is a type of sign which indicates a potential hazard, obstacle or condition requiring special attention. Some are traffic signs that indicate hazards on roads that may not be readily apparent to a driver.

General warning signs are used in instances in which the particular hazard, obstacle or condition is not covered by a standard sign. In Europe, they usually comprise an exclamation mark on the standard triangular sign with an auxiliary sign below in the local language identifying the hazard, obstacle or condition. In countries using diamond-shaped signs, the explanatory language is often written directly on the diamond-shaped sign, although it may contain only a general warning such as “Caution”, and pictograms may also be used.

Warning signs can be placed in advance of, next to or on a specific obstacle. Obstacles such as railway level crossings may have several warning signs beforehand, while bridge ramparts typically have reflective signs placed directly on them on either side. These signs can be specific to the shape requirements of the obstacle, for example, bridge rampart signs are often tall and skinny so as not to intrude into the lane.