A cartel is a group of independent market participants who collaborate with each other and avoid competing with each other in order to improve their profits and dominate the market. They seek to limit competition, fix prices, and increase prices by creating artificial shortages through low production quotas, stockpiling, and marketing quotas. Jurisdictions frequently consider cartelization to be anti-competitive behavior, leading them to outlaw or curtail cartel practices. Anti-trust law targets cartel behavior in markets.

The doctrine in economics that analyzes cartels is cartel theory. Cartels are distinguished from other forms of collusion or anti-competitive organization such as corporate mergers. Cartels are inherently unstable due to the temptation by members of the cartel to cheat and defect on each other by improving their individual profits, which may lead to falling prices for all members. Advancements in technology or the emergence of substitutes can undermine cartel pricing power, leading to the breakdown of the cooperation needed to sustain the cartel. Outside actors often respond to the undersupply of a good by bolstering their production of the good, investing in technologies that use the good more efficiently, or investing in substitutes.

Cartels can be organized on an international basis; examples of international cartels include the OPEC cartel to collude on oil production and the International Rubber Regulation Agreement to collude on rubber production. They can also exist at a national level; examples of American cartels include the United States Gunpowder Trade Association (which was dissolved by U.S. courts in 1912) and the National Collegiate Athletic Association which restricts the kind of compensation that collegiate athletes can receive.

The word cartel comes from the Italian word cartello ('leaf of paper' or 'placard'), itself derived from the Latin charta 'card'. In Middle French, the Italian word became cartel, which was borrowed into English. In English, the word was originally used for a written agreement between warring nations to regulate the treatment and exchange of prisoners from the 1690s onward. From 1899 onwards, the usage of the word became generalized as to mean any intergovernmental agreement between rival nations.

The use of the English word cartel to describe an economic group rather than international agreements was derived much later in the 1800s from the German Kartell, which also has its origins in the French cartel. It was first used between German railway companies in 1846 to describe tariff and technical standardization efforts. The first time the word was referred to describe a kind of restriction of competition was by the Austro-Hungarian political scientist Lorenz von Stein, who wrote on tariff cartels:

There's no more one-sided perspective than the one saying that such rate-cartels are "monopoly cartels" or cartels for the "exploitation of carriers".

— Lorenz von Stein, 1874

Cartels have existed since ancient times. Guilds in the European Middle Ages, associations of craftsmen or merchants of the same trade, have been regarded as cartel-like. Tightly organized sales cartels existed in the mining industry of the late Middle Ages, like the 1301 salt syndicate in France and Naples, or the Alaun cartel of 1470 between the Papal State and Naples. Both unions had common sales organizations for overall production called the Societas Communis Vendicionis ('Common Sales Society').