Surplus product (German: Mehrprodukt) is a concept theorised by Karl Marx in his critique of political economy. Roughly speaking, it is the extra goods produced above the amount needed for a community of workers to survive at its current standard of living. Marx first began to work out his idea of surplus product in his 1844 notes on James Mill's Elements of political economy.

Notions of "surplus produce" have been used in economic thought and commerce for a long time (notably by the Physiocrats), but in Das Kapital, Theories of Surplus Value and the Grundrisse Marx gave the concept a central place in his interpretation of economic history. Nowadays the concept is mainly used in Marxian economics, political anthropology, cultural anthropology, and economic anthropology.

The frequent translation of the German "Mehr" as "surplus" makes the term "surplus product" somewhat inaccurate, because it suggests to English speakers that the product referred to is "unused", "not needed", or "redundant", while most accurately "Mehr" means "more" or "added"—thus, "Mehrprodukt" refers really to the additional or "excess" product produced. In German, the term "Mehrwert" most literally means value-added, a measure of net output, (though, in Marx's particular usage, it means the surplus-value obtained from the use of capital, i.e. it refers to the net addition to the value of capital owned).

In Theories of Surplus Value, Marx says in classical economics the "surplus" referred to an excess of gross income over cost, which implied that the value of goods sold was greater than the value of the costs involved in producing or supplying them. That was how you could "make money". The surplus represented a net addition to the stock of wealth. A central theoretical question was then to explain the kinds of influences on the size of the surplus, or how the surplus originated, since that had important consequences for the funds available for re-investment, tax levies, the wealth of nations, and (especially) economic growth.

This was theoretically a confusing issue, because sometimes it seemed that a surplus arose out of clever trading in already existing assets, while at other times it seemed that the surplus arose because new value was added in production. In other words, a surplus could be formed in different ways, and one could get rich either at the expense of someone else, or by creating more wealth than there was before, or by a mixture of both. This raised the difficult problem of how, then, one could devise a system for grossing and netting incomes & expenditures to estimate only the value of the new additional wealth created by a country. For centuries, there was little agreement about that, because rival economists each had their own theory about the real sources of wealth-creation—even if they might agree that the value of production must equal the sum of the new revenue which it generates for the producers.

Political economy was originally considered to be a "moral science", which arose out of the moral and juridical ambiguities of trading processes themselves. It was analytically difficult to take the step from the incomes of individuals, the immediate source of which was rather obvious, to a consideration of the incomes of groups, social classes and nations. Somehow, a "system of transactors" showing aggregate sales and purchases, costs and incomes had to be devised, but just exactly how that system was put together, could differ a great deal, depending on "from whose point of view" the transactions were considered. The Physiocratic school, for example, believed that all wealth originated from the land, and their social accounting system was designed to show this clearly.

In Das Kapital and other writings, Marx divides the new "social product" of the working population (the flow of society's total output of new products in a defined time-interval) into the necessary product and the surplus product. Economically speaking, the "necessary" product refers to the output of products and services necessary to maintain a population of workers and their dependents at the prevailing standard of life (effectively, their total reproduction cost). The "surplus" product is whatever is produced in excess of those necessaries. Socially speaking, this division of the social product reflects the respective claims which the labouring class and the ruling class make on the new wealth created.

Strictly speaking, however, such an abstract, general distinction is a simplification, for at least three reasons.