Mustard Agent, or Sulphur Mustard, commonly known as Mustard Gas, is a class of related cytotoxic and vesicant chemical warfare agent that has the ability to form large blisters on exposed skin and in the lungs.

Pure sulphur mustards are colourless, viscous liquids at room temperature but used in impure form, such as warfare agents, they are usually yellow-brown in colour and have an odour resembling mustard plants, garlic, or horseradish, hence the name Mustard Gas.

Mustard agent was originally assigned the name LOST, after the scientists Wilhelm Lommel and Wilhelm Steinkopf, who developed a method for the large-scale production of mustard agent for the Imperial German Army in 1916.

Mustard agents are regulated under the 1993 Chemical Weapons Convention (CWC). Three classes of chemicals are monitored under this Convention, with sulphur and nitrogen mustard grouped in Schedule 1 as substances with no use other than in chemical warfare.

Mustard agents could be deployed on the battlefield by means of artillery shells, aerial bombs, rockets, or by spraying from warplanes.