Carboxymethyl cellulose (CMC) or cellulose gum is a cellulose derivative with carboxymethyl groups (-CH2-COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It is often used in its sodium salt form, sodium carboxymethyl cellulose. The addition of carboxylic acid groups to the cellulose backbone allows carboxymethyl cellulose to be dissolved in water unlike natural cellulose. This allows its use in numerous food and pharmaceutical applications that require water-soluble polymers.

Carboxymethyl cellulose is synthesized by the alkali-catalyzed reaction of cellulose with chloroacetic acid. The polar (organic acid) carboxyl groups render the cellulose soluble and chemically reactive. Fabrics made of cellulose – e.g., cotton or viscose (rayon) – may also be converted into CMC

Following the initial reaction, the resultant mixture produces approximately 60% CMC and 40% salts (sodium chloride and sodium glycolate). This product, called technical CMC, is used in detergents, where it is used as a thickening agent.An additional purification process is used to remove salts to produce pure CMC, which is used for food and pharmaceutical applications.An intermediate 'semi-purified' grade is also produced, which is typically used in paper applications such as the restoration of archival documents .