Perfluorooctanesulfonic acid (PFOS) (conjugate base perfluorooctanesulfonate) is a chemical compound having an eight-carbon fluorocarbon chain and a sulfonic acid functional group, and thus it is a perfluorosulfonic acid and a perfluoroalkyl substance (PFAS). It is an anthropogenic (man-made) fluorosurfactant, now regarded as a global pollutant. PFOS was the key ingredient in Scotchgard, a fabric protector made by 3M, and related stain repellents. The acronym "PFOS" refers to the parent sulfonic acid and to various salts of perfluorooctanesulfonate. These are all colorless or white, water-soluble solids. Although of low acute toxicity, PFOS has attracted much attention for its pervasiveness and environmental impact. It was added to Annex B of the Stockholm Convention on Persistent Organic Pollutants in May 2009.

In 1949, 3M began producing PFOS-based compounds by electrochemical fluorination. In 1968, organofluorine compounds were detected in the blood serum of consumers, and in 1976, perfluorooctanoic acid (PFOA) or a related compound such as PFOS were suggested as components. In 1997, 3M detected PFOS in blood from global blood banks, although the company's internal documents indicate knowledge of this decades earlier, dating from the 1970s. In 1999, the U.S. Environmental Protection Agency began investigating perfluorinated compounds after receiving data on the global distribution and toxicity of PFOS, the key ingredient in Scotchgard. For these reasons, and USEPA pressure, the primary American producer of PFOS, 3M, announced, in May 2000, the phaseout of the production of PFOS, PFOA, and PFOS-related products. Most other manufacturers (particularly, those in Europe) phased out the production of PFOS and perfluorooctanoic acid (PFOA) in 2000 and 2006, respectively. A shorter-chain PFOS (perfluorohexanesulfonic acid, PFHxS), was included in Annex A to the Stockholm Convention in 2022.

Currently, most of PFOS and PFOS-related chemicals are produced in China.

The main method used for the industrial scale production of PFOS is electrochemical fluorination (ECF). ECF is an electrolysis method whereby the precursor of octanesulfonyl fluoride is electrolyzed in a solution of hydrogen fluoride to give perfluorooctanesulfonyl fluoride. This production method also results in shorter chain perfluoroalkyl substances being formed.^{[citation needed]} PFOS predominates in the resultant mixture. A distinct isomer ratio has been observed in PFOS produced by ECF, in the order of 70% linear PFOS, 25% branched and 5% terminal; this is not a function of the production process but rather that the precursor also exhibits this isomer ratio. ECF was the means by which 3M produced PFOS up until May 2000 when the company announced a phaseout of fluorosurfactants.

Although 89 constitutional isomers of PFOS are possible, environmental samples usually consist of a mixture of the linear isomer and 10 branched isomers.

Telomerisation involves constructing the PFOS molecule using short chain (often 2-carbon) precursors and adding a sulfonate group as a final step. This production process results in 100% linear PFOS. This production method, whilst cleaner and resulting in a much purer product than ECF, is not known to have been widely used except for the production of reagent grade PFOS and analytical standards.^{[citation needed]}

Perfluorooctylsulfonyl compounds degrade to PFOS. Examples include N-methyl perfluorooctane sulfonamidoethanol [de] (N-MeFOSE), a carpet stain repellent, and N-ethyl perfluorooctane sulfonamidoethanol [de] (N-EtFOSE), a paper treatment. Also perfluorooctanesulfonamide is a precursor. About 50 precursors were named in the 2004 proposed Canadian ban on PFOS.

PFOS virtually does not degrade under environmental conditions and is thus highly persistent. Waste water treatment plants are also unable to degrade PFOS. On the other hand, precursors are transformed to PFOS in waste water treatment plants.