My project explores the history of rumors (ryūgen higo) from the buildup to the Russo-Japanese War to the end of the Occupation of Japan. During this period, journalists, critics, and government officials treated rumors as a form of popular political speech, a means of social control, and an object of academic study in connection with specific crises in the twentieth century. They blamed – and sometimes encouraged – rumors for causing panic around the time of the Russo-Japanese War, massacres of Korean residents after the Great Kantō Earthquake of 1923, poor home-front morale during World War II, and economic instability during the Allied Occupation. Drawing on a variety of sources from the popular press, the police, and social psychology research, I show how different efforts to report, analyze, and suppress rumors reconfigured the boundaries between political and apolitical speech during a new age of mass politics and planning in the twentieth century.