Gaetano Bresci (Italian: [ɡaeˈtaːno ˈbreʃʃi]; 11 November 1869 – 22 May 1901) was an Italian anarchist who assassinated King Umberto I of Italy. His experience of working as a young weaver led him to realize he was exploited in the workplace, which attracted him to anarchism. Bresci emigrated to the United States, where he became involved with other Italian immigrant anarchists in Paterson, New Jersey. News of the Bava Beccaris massacre motivated him to return to Italy, where he planned to assassinate Umberto in response. Local police knew of his return but did not mobilize. Bresci killed the king in July 1900 during Umberto's scheduled appearance in Monza amid a sparse police presence.

The government of Italy suspected that Bresci had been a part of a conspiracy, but no evidence was found to indicate that others were involved. He was consequently sentenced to life imprisonment for murder and confined on Santo Stefano Island in Latina, Lazio, where he was found dead of an apparent suicide within the year. After his death, Bresci gained the status of a martyr within the Italian anarchist movement, who defended his regicidal act. Bresci inspired some anarchists to carry out their own acts of propaganda by deed, most prominently Leon Czolgosz's assassination of United States President William McKinley. Italian anarchists erected a monument to Bresci in Carrara despite governmental attempts to block it.

On 11 November 1869, Gaetano Bresci was born into a lower middle-class family in Prato, Tuscany. The son of Gaspero Bresci and Maddalena Godi, his parents owned a small amount of land in Coiano, where they farmed grapes, olives, and wheat. His older brothers, Lorenzo and Angiolo, respectively worked as a shoemaker and as an officer in the Italian military. In 1880, the Kingdom of Italy began importing cheap grain from the United States, which economically devastated small farmers like the Brescis. As the price of grain fell, the family fell into poverty and Gaetano himself started working to support his family's income. He came to blame the Italian state for his family's experiences with poverty. When he was 11 years old, Bresci began an apprenticeship as a weaver at a textile factory. On Sundays, he attended a vocational school, where he specialised in weaving silk. By the time he reached the age of 15, he had qualified to work as a silk weaver.

Bresci was radicalized by his experience of being exploited in the workplace, and he joined the Italian anarchist movement. On 3 October 1892, Bresci and a group of about twenty anarchists confronted two police officers that had given a young worker a citation for not closing his butcher shop on time. Armed police dispersed the group and Bresci was later arrested for the act. On 27 December 1892, he was tried and found guilty of insulting the police. He was sentenced to 15 days in prison, and was subsequently marked in police files as a "dangerous anarchist".

Bresci was arrested again in 1895, after organising a textile workers' strike, for which he was exiled to Lampedusa by the government of Francesco Crispi. During his forced residence [it] on the island, Bresci studied anarchist literature and became further radicalized. Bresci was granted amnesty in 1896, and returned to the mainland. His status as an anarchist activist also followed him and he initially had difficulty finding work, before being hired to work at a wool factory. He developed a reputation as a dandy and engaged in numerous affairs, possibly fathering a child with one of his co-workers. Sustained economic difficulties, among other factors, soon led him to consider emigration.

In 1897, Bresci immigrated to the United States. From New York City, Bresci moved to Hoboken, New Jersey, where he met and married Sophie Kneiland, an Irish-American with whom he fathered two daughters: Madeleine and Gaetanina. To support his family, Bresci spent his weekdays working as a silk weaver in Paterson, New Jersey, returning to Hoboken on weekends. He indulged in purchases of fine clothing with his wages, and developed photography as a hobby. In Paterson, Bresci quickly became involved in the local trade unions and the immigrant anarchist movement. He briefly joined the Right to Existence Group (Italian: Gruppo diritto all'esistenza), but left after a few months as he found the group insufficiently radical. At one of the group's meetings, Bresci reportedly saved the life of Errico Malatesta, when he disarmed a disgruntled individualist anarchist who had shot and wounded Malatesta. Bresci also co-founded and financially supported its newspaper, La Questione Sociale, for which he became a prolific "firebrand" contributor. He wrote to his brother that they benefitted from freedom of the press and relative political equality in the United States, but that anti-Italian sentiments also ran high, recalling that Anglo-Americans called Italians "pigs".

In 1898, Bresci received news of the Bava Beccaris massacre. Protests in Milan against the rising price of bread had been violently suppressed by the Royal Italian Army, which fired on and killed many of the protestors. By this time, Bresci had fallen under the influence of the individualist anarchist Giuseppe Ciancabilla, an advocate of propaganda of the deed. Bresci swore revenge against King Umberto I of Italy, who he held personally responsible for the massacre as he had decreed a state of siege in Milan and awarded a medal to Fiorenzo Bava Beccaris, the general who ordered the shooting.

Bresci requested that La Questione Sociale return $150 (equivalent to $5,656 in 2024) which he had lent to them, and with it he bought a Harrington & Richardson revolver in .32 or .38 S&W and a one-way ticket back to Europe. They respectively cost him $7 (equivalent to $264 in 2024) and $27 (equivalent to $1,018 in 2024), with the latter discounted for the occasion of the 1900 Paris Exposition. Before leaving, he told his wife that he was returning to resolve his deceased parents' estate. Bresci set sail in May 1900, disembarking at Le Havre and briefly staying in Paris, before leaving for Italy. In June 1900, Bresci returned to his home city of Prato, where he stayed with his brother's family. Although the local police chief was aware of Bresci's presence and knew that police records had listed him as a "dangerous anarchist", the chief did not follow procedure of informing Italy's Ministry of the Interior or retaining Bresci's passport. Unsurveilled, Bresci was free to practice firing his revolver daily.