Mutualism is an anarchist school of thought and economic theory that advocates a socialist society based on free markets and usufructs, i.e. occupation and use property norms. One implementation of this system involves the establishment of a mutual-credit bank that would lend to producers at a minimal interest rate, just high enough to cover administration. Mutualism is based on a version of the labor theory of value that it uses as its basis for determining economic value. According to mutualist theory, when a worker sells the product of their labor, they ought to receive money, goods, or services in exchange that are equal in economic value, embodying "the amount of labor necessary to produce an article of exactly similar and equal utility". The product of the worker's labour factors the amount of both mental and physical labour into the price of their product.

While mutualism was popularized by the writings of anarchist philosopher Pierre-Joseph Proudhon and is mainly associated as an anarchist school of thought and with libertarian socialism, its origins as a type of socialism go back to the 18th-century labour movement in Britain first, then France and finally to the working-class Chartist movement and Boston Anarchists. Mutualists oppose individuals receiving income through loans, investments and rent under capitalist social relations. Although opposed to this type of income, Proudhon expressed that he had never intended "to forbid or suppress, by sovereign decree, ground rent and interest on capital. I think that all these manifestations of human activity should remain free and voluntary for all: I ask for them no modifications, restrictions or suppressions, other than those which result naturally and of necessity from the universalization of the principle of reciprocity which I propose."

As long as they ensure the worker's right to the full product of their labour, mutualists support markets and property in the product of labour, differentiating between capitalist private property (productive property) and personal property (private property). Mutualists argue for conditional titles to land, whose ownership is legitimate only so long as it remains in use or occupation (which Proudhon called possession), a type of private property with strong abandonment criteria. This contrasts with the capitalist non-proviso labour theory of property, where an owner maintains a property title more or less until one decides to give or sell it.

As libertarian socialists, mutualists distinguish their market socialism from state socialism and do not advocate state ownership over the means of production. Instead, each person possesses a means of production, either individually or collectively, with trade representing equivalent amounts of labour in the free market. Benjamin Tucker wrote of Proudhon that "though opposed to socializing the ownership of capital, he aimed nevertheless to socialize its effects by making its use beneficial to all instead of a means of impoverishing the many to enrich the few ... by subjecting capital to the natural law of competition, thus bringing the price of its own use down to cost".

Although similar to the economic doctrines of the 19th-century American individualist anarchists, mutualism favours large industries. Mutualism has been retrospectively characterized sometimes as being a form of individualist anarchism and ideologically situated between individualist and collectivist forms of anarchism. Proudhon described the liberty he pursued as "the synthesis of communism and property". Some consider mutualism part of free-market anarchism, individualist anarchism and market-oriented left-libertarianism, while others regard it as part of social anarchism.