Abstract: A pyramidal pop-open warning sign includes a support frame having a one-piece vertex with resilient arms radiating outward and downward therefrom. The upper ends of tubular legs are respectively coupled to outer ends of the arms for biased rotation between a deployed position extending downwardly and outwardly from the vertex and a contracted position disposed generally parallel to a vertical axis extending through the center of the vertex. A ground engaging foot is disposed at a lower end of each of the legs. A pyramidal skirt made of a flexible material and comprising a plurality of triangular panels, each displaying a warning message and connected together at lateral edges thereof, is draped over the support frame such that each of the panels is respectively disposed between an associated pair of adjacent legs and is stretched taut therebetween when the associated pair of legs is disposed in the deployed position.