Chloroacetic acid, industrially known as monochloroacetic acid (MCA), is the organochlorine compound with the formula ClCH₂CO₂H. This carboxylic acid is a useful building block in organic synthesis. It is a colorless solid. Related compounds are dichloroacetic acid and trichloroacetic acid.Most reactions take advantage of the high reactivity of the C−Cl bond. In its largest-scale application, chloroacetic acid is used to prepare the thickening agent carboxymethyl cellulose and carboxymethyl starch.Chloroacetic acid is also used in the production of phenoxy herbicides by etherification with chlorophenols. In this way 2-methyl-4-chlorophenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid, and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) are produced. It is the precursor to the herbicide glyphosate and dimethoate. Chloroacetic acid is converted to chloroacetyl chloride, a precursor to adrenaline (epinephrine). Displacement of chloride by sulfide gives thioglycolic acid, which is used as a stabilizer in PVC and a component in some cosmetics.Illustrative of its usefulness in organic chemistry is the O-alkylation of salicylaldehyde with chloroacetic acid, followed by decarboxylation of the resulting ether, producing benzofuran.

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Like other chloroacetic acids and related halocarbons, chloroacetic acid is a hazardous alkylating agent. The LD₅₀ for rats is 76 mg/kg.It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.