Lactic acid is a chemical compound that plays a role in several biochemical processes. It is the acid that gives old milk its sour taste, and it accumulates in skeletal muscles during extensive anaerobic exercise, causing temporary muscle pain. Lactic acid is quickly removed from muscles when they resume aerobic metabolism. Delayed onset muscle soreness usually becomes apparent more than 24 hours after exercising and is not caused by lactic acid buildup. Lactic acid is a carboxylic acid and its chemical formula is C₃H₆O₃ and its structure is reflected in its systematic name, 2-hydroxypropanoic acid.

In solution, it can lose a proton from the COOH carboxy group, turning into the lactate ion CH₃CHOHCOO^-.

There are two optical isomers of lactic acid (and of lactate) since the central carbon atom is bound to four different groups. The first isomer is known as L(+)-lactic acid or (S)-lactic acid and the second is D(-)-lactic acid or (R)-lactic acid.

During one form of anerobic glycolysis or fermentation, L-lactate is produced from pyruvate via the enzyme lactate dehydrogenase. This conversion also oxidizes one molecule of NADH to NAD⁺, and this is the reason for the conversion: NAD⁺ has to be regenerated so that glycolysis can continue.

This lactic acid fermentation occurs in red blood cells since they don't have mitochondria, and in skeletal muscle during intense exertion when sufficient amounts of oxygen cannot be supplied fast enough. This lactate is released into the bloodstream. The typical lactate concentration in the blood is 1-2 mmole/L. The liver takes up about 60% of the lactate and reoxidizes it to pyruvate, which is then reconverted to glucose in a process known as gluconeogenesis. The glucose enters the bloodstream and can be used by the tissues. This glucose → lactate → glucose cycle, originally described by Carl and Gerty Cori, is known as the Cori cycle. About 40% of the lactate is taken up by well oxygenated muscle cells and oxidized to pyruvate, which is then directly used to fuel the citric acid cycle.

This lactic acid fermentation is also performed by lactic acid bacteria which are responsible for the sour taste of old milk and which are used in the production of dairy products such as cheese, yoghurt and kefir. Lactic acid fermentation also gives the sour taste to fermented vegetables such as traditionally cultured sauerkraut and pickles and many fermented starches such as poi. Lactic acid bacteria also operates in the mouth, and the resulting lactic acid is responsible for the tooth decay known as caries.

Lactic acid is also the result of malolactic fermentation, a process used in winemaking to convert sharp-tasting malic acid into the gentler lactic acid.

The CAS number of lactic acid is 50-21-5. To distinguish between the optical isomers, one can use CAS 79-33-4 for L-lactic acid and CAS 10326-41-7 for D-lactic acid and CAS 598-82-3 for an optically inactive mixture of the two.