The iron law of oligarchy is a political theory first developed by the German-born Italian sociologist Robert Michels in his 1911 book Political Parties. It asserts that rule by an elite, or oligarchy, is inevitable as an "iron law" within any democratic organization as part of the "tactical and technical necessities" of the organization.

Michels's theory states that all complex organizations, regardless of how democratic they are when started, eventually develop into oligarchies. Michels observed that since no sufficiently large and complex organization can function purely as a direct democracy, power within an organization will always get delegated to individuals within that group, elected or otherwise.

Using anecdotes from political parties and trade unions struggling to operate democratically to build his argument in 1911, Michels addressed the application of this law to representative democracy, and stated: "Who says organization, says oligarchy." He went on to state that "Historical evolution mocks all the prophylactic measures that have been adopted for the prevention of oligarchy."

According to Michels, all organizations eventually come to be run by a "leadership class", who often function as paid administrators, executives, spokespersons or political strategists for the organization. Far from being "servants of the masses", Michels argues this "leadership class", rather than the organization's membership, will inevitably grow to dominate the organization's power structures. By controlling who has access to information, those in power can centralize their power successfully, often with little accountability, due to the apathy, indifference and non-participation most rank-and-file members have in relation to their organization's decision-making processes. Michels argues that democratic attempts to hold leadership positions accountable are prone to fail, since with power comes the ability to reward loyalty, the ability to control information about the organization, and the ability to control what procedures the organization follows when making decisions. All of these mechanisms can be used to strongly influence the outcome of any decisions made 'democratically' by members.

Michels stated that the official goal of representative democracy of eliminating elite rule was impossible, that representative democracy is a façade legitimizing the rule of a particular elite, and that elite rule, which he refers to as oligarchy, is inevitable. Later Michels migrated to Italy and joined Benito Mussolini's Fascist Party, as he believed this was the next legitimate step of modern societies. The thesis became popular once more in post-war America with the publication of Union Democracy: The Internal Politics of the International Typographical Union (1956) and during the red scare brought about by McCarthyism.