Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is any chemical compound that contains the element chromium in the +6 oxidation state (thus hexavalent). It has been identified as carcinogenic, which is of concern since approximately 136,000 tonnes (150,000 tons) of hexavalent chromium were produced in 1985. Hexavalent chromium compounds can be carcinogens (IARC Group 1), especially if airborne and inhaled where they can cause lung cancer.

Hexavalent chromium occurs only rarely in nature, an exception being crocoite (PbCrO₄). It is however produced on a large scale industrially. Virtually all chromium ore is processed via the formation of hexavalent chromium, specifically the salt sodium dichromate. Sodium chromate is converted into other hexavalent chromium compounds such as chromium trioxide and various salts of chromate and dichromate.

Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics; chromates added as anticorrosive agents to paints, primers, and other surface coatings; and chromic acid electroplated onto metal parts to provide a decorative or protective coating.

Hexavalent chromium indeed is one of the most widely used heavy metals in various sectors and industries (metallurgy, chemicals, textiles, etc.) with particular involvement in the metal coating sector, especially when subjected to plating or coating processes involving hexavalent chromium.

Hexavalent chromium can be formed when performing "hot work" such as welding on stainless steel or melting chromium metal. In these situations the chromium is not originally hexavalent, but the high temperatures involved in the process result in oxidation that converts the chromium to a hexavalent state. Hexavalent chromium can also be found in drinking water and public water systems.

Many hexavalent chromium compounds can be carcinogens (IARC Group 1), especially if airborne and inhaled where they can cause lung cancer. Positive associations have also been observed between exposure to chromium(VI) compounds and cancer of the nose and nasal sinuses. Workers in many occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products and those who grind or weld stainless steel. Workers who are exposed to hexavalent chromium are at increased risk of developing lung cancer, asthma, or damage to the nasal epithelia and skin. Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive and the European Union regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals.

Hexavalent chromium compounds can be genotoxic carcinogens. Due to its structural similarity to sulfate, chromate (a typical form of chromium(VI) at neutral pH) is transported into cells via sulfate channels. Inside the cell, hexavalent chromium(VI) is reduced first to pentavalent chromium(V) then to trivalent chromium(III) without the aid of any enzymes. The reduction occurs via direct electron transfer primarily from ascorbate and some nonprotein thiols. Vitamin C and other reducing agents combine with chromate to give chromium(III) products inside the cell. The resultant chromium(III) forms stable complexes with nucleic acids and proteins. This causes strand breaks and Cr–DNA adducts which are responsible for mutagenic damage. According to Shi et al., the DNA can also be damaged by hydroxyl radicals produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell, which can cause double-strand breakage.

Both insoluble salts of lead and barium chromates as well as soluble chromates were negative in the implantation model of lung carcinogenesis. Yet, soluble chromates are a confirmed carcinogen so it would be prudent to consider all chromates carcinogenic.