Carlos Castillo Armas (locally ['kaɾlos kas'tiʝo 'aɾmas]; 4 November 1914 – 26 July 1957) was a Guatemalan military officer and politician who was the 28th president of Guatemala, serving from 1954 to 1957 after taking power in a coup d'état. A member of the far-right National Liberation Movement (MLN) party, his authoritarian government was closely allied with the United States.

Born to a planter, out of wedlock, Castillo Armas was educated at Guatemala's military academy. A protégé of Colonel Francisco Javier Arana, he joined Arana's forces during the 1944 uprising against President Federico Ponce Vaides. This began the Guatemalan Revolution and the introduction of representative democracy to the country. Castillo Armas joined the General Staff and became director of the military academy. Arana and Castillo Armas opposed the newly elected government of Juan José Arévalo; after Arana's failed 1949 coup, Castillo Armas went into exile in Honduras. Seeking support for another revolt, he came to the attention of the US Central Intelligence Agency (CIA). In 1950 he launched a failed assault on Guatemala City, before escaping back to Honduras. Influenced by lobbying by the United Fruit Company and Cold War fears of communism, in 1952 the US government of President Harry Truman authorized Operation PBFortune, a plot to overthrow Arévalo's successor, President Jacobo Árbenz. Castillo Armas was to lead the coup, but the plan was abandoned before being revived in a new form by US President Dwight D. Eisenhower in 1953.

In June 1954, Castillo Armas led 480 CIA-trained soldiers into Guatemala, backed by US-supplied aircraft. Despite initial setbacks to the rebel forces, US support for the rebels made the Guatemalan army reluctant to fight, and Árbenz resigned on 27 June. A series of military juntas briefly held power during negotiations that ended with Castillo Armas assuming the presidency on 7 July. Castillo Armas consolidated his power in an October 1954 election, in which he was the only candidate; the MLN, which he led, was the only party allowed to contest the congressional elections. Árbenz's popular agricultural reform was largely rolled back, with land confiscated from small farmers and returned to large landowners. Castillo Armas cracked down on unions and peasant organizations, arresting and killing thousands. He created a National Committee of Defense Against Communism, which investigated over 70,000 people and added 10 percent of the population to a list of suspected communists.

Despite these efforts, Castillo Armas faced significant internal resistance, which was blamed on communist agitation. The government, plagued by corruption and soaring debt, became dependent on aid from the US. In 1957 Castillo Armas was assassinated by a presidential guard with leftist sympathies. He was the first of a series of authoritarian rulers in Guatemala who were close allies of the US. His reversal of the reforms of his predecessors sparked a series of leftist insurgencies in the country after his death, culminating in the Guatemalan Civil War of 1960 to 1996.

Carlos Castillo Armas was born on 4 November 1914, in Santa Lucía Cotzumalguapa in the department of Escuintla. He was the son of a landowner, but was born out of wedlock, making him ineligible to inherit the property. In 1936 he graduated from the Guatemalan military academy. His time at the academy overlapped with that of Jacobo Árbenz, who would later become President of Guatemala.

In June 1944, a series of popular protests forced the resignation of dictator Jorge Ubico. Ubico's successor Federico Ponce Vaides pledged to hold free elections, but continued to suppress dissent, leading progressives in the army to plot a coup against him. The plot was initially led by Árbenz and Aldana Sandoval; Sandoval persuaded Francisco Javier Arana, the influential commander of the Guardia de Honor, to join the coup in its final stages. On 19 October, Arana and Árbenz launched a coup against Ponce Vaides' government. In the election that followed, Juan José Arévalo was elected president. Castillo Armas was a strong supporter and protégé of Arana, and thus joined the rebels. Speaking of Castillo Armas, Árbenz would later say that he was "modest, brave, sincere" and that he had fought with "great bravery" during the coup.

For seven months, between October 1945 and April 1946, Castillo Armas received training at Fort Leavenworth, Kansas, where he came in contact with American intelligence officers. After serving on the General Staff, he became director of the military academy until early 1949, at which point he was made the military commander at Mazatenango, Suchitepéquez, a remote military garrison. Castillo Armas had eventually risen to the rank of lieutenant colonel. He was at Mazatenango when Arana launched his failed coup attempt against Arévalo on 18 July 1949, and was killed: Castillo Armas did not hear of the revolt until four days later. Historians differ on what happened to him at this point. Historian Piero Gleijeses writes that Castillo Armas was expelled from the country following the coup attempt against Arévalo. Nick Cullather and Andrew Fraser state that Castillo Armas was arrested in August 1949, that Árbenz had him imprisoned under doubtful charges until December 1949, and that he was found in Honduras a month later.

Following the end of Arévalo's highly popular presidency in 1951, Árbenz was elected president. He continued the reforms of Arévalo and also began an ambitious land reform program known as Decree 900. Under it, the uncultivated portions of large land-holdings were expropriated in return for compensation and redistributed to poverty-stricken agricultural laborers. The agrarian reform law angered the United Fruit Company, which at the time dominated the Guatemalan economy. Benefiting from decades of support from the US government, by 1930 the company was already the largest landowner and employer in Guatemala. It was granted further favors by Ubico, including 200,000 hectares (490,000 acres) of public land, and an exemption from all taxes.