any of a class of organic compounds having molecular structures in which the nitroso group (-N=O) is attached to a carbon or nitrogen atom. Substances in which this group is attached to an oxygen atom are called nitrites (nitrite), that is, esters of nitrous acid; those in which the nitroso group is attached to a metal ion are called nitrosyls.

Nitroso compounds are usually prepared by the action of nitrous acid or a derivative of it upon a substance containing an easily replaced hydrogen atom. Certain members of the class are obtainable by oxidation of amines or by reduction of nitro compounds.

Examples of nitroso compounds are nitrosodimethylaniline and the nitrosophenols, used in the manufacture of dyes (dye). The compounds are usually blue or green in colour. Nitroso derivatives of amides decompose upon heating with formation of nitrogen and can be used as foam-producing agents; if they are heated in the presence of alkalies, the decomposition takes a different course, yielding diazo compounds.