A traffic warning sign is a type of traffic sign that indicates a hazard ahead on the road that may not be readily apparent to a driver.

In most countries, they usually take the shape of an equilateral triangle with a white background and a thick red border. However, both the color of the background and the color and thickness of the border varies from country to country.

In the People's Republic of China (except for Macau and Hong Kong), warning signs appear with a black border and a yellow background. In Sweden, Serbia, Bosnia and Herzegovina, Croatia, Greece, Finland, Iceland, the Republic of Macedonia and Poland, they have a red border with an amber background. This is due to the weather, as it is easier to see a red/amber sign in the snowy weather than a red/white sign. The polar bear warning sign in Svalbard recently changed from displaying a black bear on white background to a white bear on black background (both signs are triangular with a red border). Some countries (like France, Norway, Spain) that normally use a white background have adopted an orange or amber background for road work or construction signs.

Warning signs in some countries have a diamond shape in place of the standard triangular shape. In the United States, Canada, Mexico, Thailand, Australia, Japan, Philippines, Indonesia, Malaysia, New Zealand, most of South America, and also Ireland (diverging from the standards of the rest of Europe) use warning signs are black on a yellow background and usually diamond-shaped, while temporary signs (which are typically construction signs) are black on an orange background. Some other countries also use these standards for some signage.

The warning signs usually contain a symbol. In Europe they are based on the UNECE Vienna Convention on Road Signs and Signals. In United States they are based on their own standard and often contain text only.