Ascorbic acid is an organic compound with formula C₆H₈O₆, originally called hexuronic acid. It is a white solid, but impure samples can appear yellowish. It dissolves freely in water to give mildly acidic solutions. It is a mild reducing agent.

Ascorbic acid exists as two enantiomers (mirror-image isomers), commonly denoted "l" (for "levo") and "d" (for "dextro"). The l isomer is the one most often encountered: it occurs naturally in many foods, and is one form ("vitamer") of vitamin C, an essential nutrient for humans and many animals. Deficiency of vitamin C causes scurvy, formerly a major disease of sailors in long sea voyages. It is used as a food additive and a dietary supplement for its antioxidant properties. The "d" form (erythorbic acid) can be made by chemical synthesis, but has no significant biological role.

The term ascorbic means antiscurvy and denotes the ability to fight off scurvy. It is related to combating Vitamin C deficiency.

The antiscorbutic properties of certain foods were demonstrated in the 18th century by James Lind. In 1907, Axel Holst and Theodor Frølich discovered that the antiscorbutic factor was a water-soluble chemical substance, distinct from the one that prevented beriberi. Between 1928 and 1932, Albert Szent-Györgyi isolated a candidate for this substance, which he called "hexuronic acid", first from plants and later from animal adrenal glands. In 1932 Charles Glen King confirmed that it was indeed the antiscorbutic factor.

In 1933, sugar chemist Walter Norman Haworth, working with samples of "hexuronic acid" that Szent-Györgyi had isolated from paprika and sent him in the previous year, deduced the correct structure and optical-isomeric nature of the compound, and in 1934 reported its first synthesis. In reference to the compound's antiscorbutic properties, Haworth and Szent-Györgyi proposed to rename it "a-scorbic acid" for the compound, and later specifically l-ascorbic acid. Because of their work, in 1937 two Nobel Prizes: in Chemistry and in Physiology or Medicine were awarded to Haworth and Szent-Györgyi, respectively.

Ascorbic acid is a furan-based lactone of 2-ketogluconic acid. It contains an adjacent enediol adjacent to the carbonyl. This −C(OH)=C(OH)−C(=O)− structural pattern is characteristic of reductones, and increases the acidity of one of the enol hydroxyl groups. The deprotonated conjugate base is the ascorbate anion, which is stabilized by electron delocalization that results from resonance between two forms:

For this reason, ascorbic acid is much more acidic than would be expected if the compound contained only isolated hydroxyl groups.

The ascorbate anion forms salts, such as sodium ascorbate, calcium ascorbate, and potassium ascorbate.