Realgar, a-As4S4, is an arsenic sulfide mineral. It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral, orpiment (As2S3). It is orange-red in colour, melts at 320 °C, and burns with a bluish flame releasing fumes of arsenic and sulfur. Realgar is soft with a Mohs hardness of 1.5 to 2 and has a specific gravity of 3.5. Its molecular weight is 106.99. Its streak is orange colored. Realgar has a sub-metallic luster.

Its name comes from Arabic rahj al-ghar – 'powder of the mine,' via Catalan, Middle Latin, and Middle English, though this comes through a misreading, the original name was rahj al-far – 'rat powder', due to its use as a rodenticide. In India, realgar is known as manseel, and orpiment as hartal.

It was, along with orpiment, a significant item of trade in the ancient Roman Empire and was used as a pigment and a medicine. It was also used as a medicine in China and "is made up into household ornaments, such as wine pots, wine cups, images, paperweights, and various other kinds of ornaments and charms, to be kept near at hand in use, or worn about the person, with a view of warding off disease."

It is commonly held that after a long period of exposure to light realgar changes form to a yellow powder known as pararealgar (ß-As4S4). It was once thought that this powder was the yellow sulfide orpiment, but has been recently shown to be a distinct chemical compound.

Realgar, orpiment, and arsenopyrite provide nearly all the world's supply of arsenic as a byproduct of smelting concentrates derived from these ores.

On long exposure to light, realgar disintegrates into a reddish-yellow powder, requiring that specimens be protected from light exposure.Realgar is also used by firework manufacturers to create the color white in fireworks, manufacturing shot, printing and dyeing calico, and depilating and tanning hides.