Leon Frank Czolgosz (Polish: Czołgosz) (January 1, 1873–October 29, 1901) was the assassin of U.S. President William McKinley.

One of seven children of Russian-Polish immigrants, Czolgosz (pronounced cholgus) was born in Detroit, Michigan, and lived in conditions of brutal poverty most of his life. He left his family farm in Cleveland, Ohio at age ten to work at American Steel and Wire Company with two of his brothers. When Leon was 12, his mother died while giving birth to another child. At the height of his employment, he was making $4 a day, but after the workers of his factory went on strike (with no union to protect them), they were fired.

In 1898, after witnessing a series of similar strikes (many ending in police confrontation), Leon suffered a mental breakdown and had to return home, where he was constantly at odds with his family's Roman Catholic beliefs and with his stepmother. He became a recluse, and spent much of his time alone, reading socialist and anarchist newspapers. He frequented the speeches of Emma Goldman, who later wrote a piece justifying Czolgosz's assassination of McKinley. However, Czolgosz failed to ingratiate himself with any group. Indeed, his fanaticism aroused their suspicions; some even thought he may have been a covert government agent.

Czolgosz's experiences had convinced him there was a great injustice in American society, an inequity which allowed the wealthy to enrich themselves by exploiting the poor. He concluded the reason for this was the structure of government itself. Then on July 29, 1900, King Umberto I was assassinated by avowed anarchist Gaetano Bresci. Bresci told the press he had to take matters into his own hands for the sake of the common man. The assassination sent shockwaves through the American anarchist movement. In Bresci, Czolgosz found his hero: a man who had the courage to sacrifice himself for the cause. When he was later arrested, police found a folded newspaper clipping about Bresci in Czolgosz’s pocket.

On August 31, 1901 he moved to Buffalo, New York and rented a room near the site of the Pan-American Exposition. On September 6, Czolgosz went to the exposition with a pistol in his hand, concealed in a bandage. McKinley had been standing in a receiving line greeting the public for several minutes when Czolgosz shot McKinley twice at point-blank range. McKinley would die from his wounds on September 14.

After a short trial, Czolgosz was found guilty and executed by electrocution, by three jolts at 1700 volts each, on October 29, 1901, in Auburn prison in Auburn, New York. His last words were "I killed the President because he was the enemy of the good people—the good working people. I am not sorry for my crime."

The gun used by Czolgosz was an Iver-Johnson "Safety Automatic" revolver in .32 S&W caliber, serial number 463344. Czolgosz purchased the gun on September 2, 1901.

His story, along with those of 8 other presidential assassins and would-be assassins, was the basis of Stephen Sondheim-John Weidman Broadway musical Assassins.