The first Red Scare was a period during the early 20th-century history of the United States marked by a widespread fear of far-left movements, including Bolshevism and anarchism, due to real and imagined events; real events included the Russian 1917 October Revolution and anarchist bombings. At its height in 1919–1920, concerns over the effects of radical political agitation in American society and the alleged spread of socialism, communism and anarchism in the American labor movement fueled a general sense of concern.

The scare had its origins in the hyper-nationalism of World War I as well as the Russian Revolution. At the war's end, following the October Revolution, American authorities saw the threat of communist revolution in the actions of organized labor, including such disparate cases as the Seattle General Strike and the Boston Police Strike and then in the bombing campaign directed by anarchist groups at political and business leaders. Fueled by labor unrest and the anarchist bombings, and then spurred on by the Palmer Raids and attempts by United States Attorney General A. Mitchell Palmer to suppress radical organizations, it was characterized by exaggerated rhetoric, illegal search and seizures, unwarranted arrests and detentions, and the deportation of several hundred suspected radicals and anarchists. In addition, the growing anti-immigration nativist movement among Americans viewed increasing immigration from Southern Europe and Eastern Europe as a threat to American political and social stability.

Bolshevism and the threat of a communist-inspired revolution in the U.S. became the overriding explanation for challenges to the social order, even for such largely unrelated events as incidents of interracial violence during the Red Summer of 1919. Fear of radicalism was used to explain the suppression of freedom of expression in the form of display of certain flags and banners. In April 1920, concerns peaked with J. Edgar Hoover telling the nation to prepare for a bloody uprising on May Day. Police and militias prepared for the worst, but May Day passed without incident. Soon, public opinion and the courts turned against Palmer, putting an end to his raids and the first Red Scare. Following the end of the Cold War, declassified documents revealed extensive Soviet spy activity in the United States as early as the 1920s.