Surplus product (German: Mehrprodukt) is an economic concept explicitly 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).