Darier's sign is a change observed after stroking the skin of a person with systemic mastocytosis or urticaria pigmentosa.

Darier's sign is named after the French dermatologist Ferdinand-Jean Darier who first described it.

Manipulation of the tumor may result in redness and swelling from release of mast cell granules, also known as Darier's sign, and prolonged local hemorrhage.

These mast cells, when irritated (e.g. by rubbing the skin, heat exposure), produce too much histamine, triggering an allergic reaction that leads to hives localized to the area of irritation, sometimes referred to as Darier's sign.