Sesquimustard (military code Q) is the organosulfur compound with the formula (ClCH2CH2SCH2)2.

The compound is a type of mustard gas, a vesicant used as a chemical weapon.

From the chemical perspective, the compound is both a thioether and an alkyl chloride.

Because sesquimustard is a solid at room temperature, it is not as easily deployed as related liquid mustards.

It was only ever deployed as mixtures with the original mustard, with phosgene, or as a solution.