Abstract: The public trust doctrine originated—and has persisted in American law—as antimonopoly protection. From the time of its recognition by American courts in the early nineteenth century, the doctrine has protected the public against private monopolization of natural resources, beginning with tidal waters and wild animals. Ensuing public trust case law has extended the scope of trust protection to other important natural resources, including non-tidal and non-navigable waters, and land-based resources like parks. Courts are now considering the trust doctrine’s application to the atmosphere. Although there is a considerable body of legal scholarship on the public trust, the doctrine’s antimonopoly core has not been explored. In this Article, we remedy that oversight by examining the public trust’s justification as an antimonopoly sentiment. Antimonopoly policy is at least as old in American law as the public trust and certainly more politically prominent. Viewing the public trust through the lens of antimonopoly helps to explain the history and evolution of this doctrine and its overriding goal of preventing irreversible commitments of natural resources to private monopolization.