In trade, barter (derived from bareter) is a system of exchange in which participants in a transaction directly exchange goods or services for other goods or services without using a medium of exchange, such as money. Barter is considered one of the earliest systems of economic exchange, used before the invention of money. Economists usually distinguish barter from gift economies in many ways; barter, for example, features immediate reciprocal exchange, not one delayed in time. Barter usually takes place on a bilateral basis, but may be multilateral (if it is mediated through a trade exchange). In most developed countries, barter usually exists parallel to monetary systems only to a very limited extent. Market actors use barter as a replacement for money as the method of exchange in times of monetary crisis, such as when currency becomes unstable (such as hyperinflation or a deflationary spiral) or simply unavailable for conducting commerce.

No ethnographic studies have shown that any present or past society has used barter without any other medium of exchange or measurement, and anthropologists have found no evidence that money emerged from barter. Nevertheless, economists since the times of Adam Smith (1723–1790) often imagined pre-modern societies for the sake of showing how the inefficiency of barter explains the emergence of money and the economy, and hence the discipline of economics itself.

Adam Smith sought to demonstrate that markets (and economies) pre-existed the state. He argued that money was not the creation of governments. Markets emerged, in his view, out of the division of labour, by which individuals began to specialize in specific crafts and hence had to depend on others for subsistence goods. These goods were first exchanged by barter. Specialization depended on trade but was hindered by the "double coincidence of wants" which barter requires, i.e., for the exchange to occur, each participant must want what the other has. To complete this hypothetical history, craftsmen would stockpile one particular good, be it salt or metal, that they thought no one would refuse. This is the origin of money according to Smith. Money, as a universally desired medium of exchange, allows each half of the transaction to be separated.

Barter is characterized in Adam Smith's "The Wealth of Nations" by a disparaging vocabulary: "haggling, swapping, dickering". It has also been characterized as negative reciprocity, or "selfish profiteering".

Anthropologists such as David Graeber have argued, in contrast, "that when something resembling barter does occur in stateless societies it is almost always between strangers." Barter occurred between strangers, not fellow villagers, and hence cannot be used to naturalistically explain the origin of money without the state. Since most people who engaged in trade knew each other, exchange was fostered through the extension of credit. Marcel Mauss, author of 'The Gift', argued that the first economic contracts were not to act in one's economic self-interest, and that before money, exchange was fostered through the processes of reciprocity and redistribution, not barter. Everyday exchange relations in such societies are characterized by generalized reciprocity, or a non-calculative familial "communism" where each takes according to their needs, and gives as they have.

Often the following features are associated with barter transactions:

There is a demand for things of a different kind.

The parties of the barter transaction are both equal and free.