The Preparedness Day Bombing was a bombing in San Francisco, California, United States, on July 22, 1916, of a parade organised by local supporters of the Preparedness Movement which advocated American entry into World War I. During the parade a suitcase bomb was detonated, killing ten and wounding 40 in the worst terrorist attack in San Francisco's history.

Two labor leaders, Thomas Mooney and Warren K Billings, were convicted in separate trials and sentenced to death, later commuted to life in prison. Later investigations found the convictions to have been based on false testimony, and the men were released in 1939 and eventually pardoned. The identity of the bombers has never been determined.