Mustard gas or sulfur mustard is a chemical compound belonging to a family of cytotoxic and blister agents known as mustard agents. The name mustard gas is widely used, but it is technically incorrect: the substance, when dispersed, is often not actually in a vapor, but is instead in the form of a fine mist of liquid droplets.Mustard gas has a long history of being used as a blister agent in warfare and is one of the most well-studied of such agents. It can form large blisters on exposed skin and in the lungs, often resulting in prolonged illness ending in death. Sulfur mustards are viscous liquids at room temperature and have an odor resembling mustard plants, garlic, or horseradish, hence the name. When pure, they are colorless, but when used in impure forms, such as in warfare, they are usually yellow-brown.

As a chemical weapon, mustard gas was first used in World War I, and has been used in several armed conflicts since then, including the Iran–Iraq War, resulting in more than 100,000 casualties. Mustard agents are regulated under the 1993 Chemical Weapons Convention. Three classes of chemicals are monitored under this convention, with sulfur and nitrogen mustard grouped in Schedule 1, as substances with no use other than in chemical warfare (though since then, mustard gas has been found to be useful in cancer chemotherapy). Mustard agents could be deployed by means of artillery shells, aerial bombs, rockets, or by spraying from warplanes or other aircraft.