The "marketplace of ideas" is a rationale for freedom of expression based on an analogy to the economic concept of a free market. The "marketplace of ideas" belief holds that the truth or the best policy arises out of the competition of widely various ideas in free, transparent public discourse, an important part of liberal democracy.

The concept of the "marketplace of ideas" was first explicitly articlulated in the dissenting opinion of Associate Justice Oliver Wendell Holmes, Jr. (joined by Associate Justice Louis Brandeis) in the U.S. Supreme Court case Abrams v. U.S. (1919).

Persecution for the expression of opinions seems to me perfectly logical. If you have no doubt of your premises or your power and want a certain result with all your heart you naturally express your wishes in law and sweep away all opposition...But when men have realized that time has upset many fighting faiths, they may come to believe even more than they believe the very foundations of their own conduct that the ultimate good desired is better reached by free trade in ideas—that the best test of truth is the power of the thought to get itself accepted in the competition of the market, and that truth is the only ground upon which their wishes safely can be carried out. That at any rate is the theory of our Constitution.