Warning signs signal workplace hazard levels that require extra awareness and could result in a serious injury or death but do not pose an immediate threat to the degree of a danger zone.

Warning signs also contain their own distinct surround shapes — an equilateral rotated square, with lines sloping at a diagonal 45-degrees as opposed to straight, 90-degree sides on a horizontal base. Warning signs can additionally contain a black exclamation mark station inside a triangle pictogram, printed to the right of the "Warning" signal word.

Warning signs also come in yellow, with additional black-lined pictograms indicating the behavior, object or activity requiring precautions.