The siege of Madrid was a two-and-a-half-year siege of the Republican-controlled Spanish capital city of Madrid by the Nationalist armies, under General Francisco Franco, during the Spanish Civil War (1936–1939). The city, besieged from October 1936, fell to the Nationalist armies on 28 March 1939. The Battle of Madrid in November 1936 saw the most intense fighting in and around the city when the Nationalists made their most determined attempt to take the Republican capital.

The highest military awards of the Spanish Republic, the Laureate Plate of Madrid (Spanish: Placa Laureada de Madrid), and the Madrid Distinction (Spanish: Distintivo de Madrid), established by the Republican government to reward courage, were named after the capital of Spain because the city symbolised valour and Republican resistance during the long siege throughout the war.

The Spanish Civil War began with a coup d'état against the Popular Front Government of the Spanish Republic by Spanish Army officers on 18 July 1936.

In Madrid, the Republican government was unsure what to do. It wanted to put down the coup but did not know if it could trust the armed forces, and it did not want to arm the Confederación Nacional del Trabajo (CNT) and Unión General de Trabajadores (UGT) trade unions and potentially accelerate the ongoing Spanish Revolution. On 18 July, the government sent units of the Guardia Civil to Seville to put down the rebellion there. However, on reaching the city, the guardias defected to the insurgents. On 19 July, Santiago Casares Quiroga resigned as prime minister and was succeeded by Diego Martinez Barrio, who tried to arrange a truce with the insurgent General Emilio Mola by telephone, but Mola refused the offer and Martinez Barrio was ousted by José Giral. Giral agreed to arm the trade unionists to defend the Republic and had 60,000 rifles delivered to the CNT and UGT headquarters although only 5,000 were in working order. In a radio broadcast on the 18th, the communist leader Dolores Ibarruri coined the famous slogan ¡No pasarán! ("They shall not pass") to urge resistance against the coup. The slogan was to become synonymous with the defense of Madrid and the Republican cause in general.

General Joaquín Fanjul, the commander of the military garrison based in Montaña barracks in Madrid, was preparing to launch the military rebellion in the city. However, when he tried to march out of the barracks, his 2,500 troops were forced back inside the compound by hostile crowds and armed trade unionists. On the 20th, the barracks was stormed by a mixture of workers and Guardias de Asalto ("assault guards", an urban police force) loyal to the government, as well as five battalions of the communist-led Antifascist Worker and Peasant Militias (MAOC). One of the battalions became the famous "Fifth Regiment" of about 10,000 fighters. The fighting was chaotic, and on several occasions, some soldiers in the barracks indicated their willingness to surrender, only for other troops to keep firing at the attackers, which killed those who had broken cover to take them prisoner.^{[citation needed]}

Eventually, the barracks fell when the Guardias de Asalto brought up a 75 mm field gun to bombard the complex and its gate was opened by a sapper sergeant sympathetic to the Republicans. The sergeant was killed by one of his officers, but his action allowed the Republicans to breach the walls. Many soldiers were massacred by the crowd, which had been enraged by the apparent false surrenders, after the fall of the barracks.

For the remainder of the war, Madrid was held by the Republicans. However, its population contained a significant number of Nationalist sympathisers, over 20,000 of which sought refuge in foreign embassies in the city. The weeks that followed the July uprising saw a number of so-called fascists or fascist sympathisers being killed in Madrid by Republicans. For example, on 23 August, seventy prisoners from the Modelo Prison in the city were massacred in revenge for the Nationalist killing of over 1,500 Republicans after the storming of Badajoz.

The initial strategy of the military plot had been to assume power all over the country in the manner of a pronunciamiento (military coup) of the 19th century. However, resistance to the coup by Republicans meant that Francisco Franco and his allies instead had to conquer the country by military force if they wanted to seize power. Franco himself had landed in Algeciras, in southern Spain, with Moroccan troops from the Spanish Army of Africa. Mola, who commanded the colonial troops as well as the Spanish Foreign Legion and Carlist and Falangist militia, raised troops in the north. Together, they planned a "Drive on Madrid" with Franco advancing from Badajoz, which he took in August and Mola advancing from Burgos. Franco's veteran colonial troops, or regulares, under General Juan Yagüe, along with air cover supplied by Nazi Germany, routed the Republican militias in their path. Yague argued for a rapid advance on Madrid, but Franco overruled him to relieve the Nationalist troops besieged in Toledo. That diversion held up the attack on Madrid by up to a month, which gave the Republicans time to prepare its defence.