The Madrid Defense Council (Spanish: Junta de Defensa de Madrid) was an ad-hoc governing body that ran Madrid, Spain, for about six months during the Spanish Civil War (1936–39). It was formed in November 1936 after the Spanish Republican government had fled to Valencia when General Francisco Franco's forces advanced on Madrid. It was expected that the city would fall within a few days, but the arrival of the International Brigades halted the rebel advance, and the situation settled into a stalemate. The council was dominated by communists, who had superior organization and propaganda to the other groups. Their policy was to organize the militias into regular troops and focus on defeating the enemy, rather than to undertake revolutionary activity. As time passed there was growing tension between the communists and more radical groups. The council was dissolved in April 1937 and replaced by a new city council.

At the start of November 1936 the rebel armies led by General Francisco Franco approached Madrid, brushing aside the untrained and ill-equipped militias they encountered. The government of the Second Spanish Republic under Francisco Largo Caballero had done nothing to prepare the capital's defenses for fear of alarming the population. The government fled from Madrid to Valencia on 6 November 1936, leaving General José Miaja to govern the city and mobilize civilians for defense. Miaja was told to defend the capital "at all costs", an apparently hopeless assignment. In sealed orders, which Miaja opened after the politicians had left, he was given authority to create a Defense Council.

The international press reported that Madrid would not be defended, and General Franco declared prematurely that the city had been liberated. Radio Lisbon even reported Franco's victorious entry into the city. On 7 November the first troops of Franco's Army of Africa reached the Casa de Campo on the edge of the city. Formation of the Madrid Defense Council was announced that day,

By mandate of the political and trade union organizations, and in accordance with the order of the Government of the Republic, there has been constituted in Madrid a Junta of Defense. It is made up of all the organizations which form part of the Government and which contribute their effort to the struggle taking place at the gates of Madrid.

The Defense Council was desperately short of men and munitions. However, Franco's forces were over-extended, with only 3,000 men available to push into the city, and Franco was naturally cautious. The first units of the International Brigades reached the city on 8 November 1936 and were immediately thrown into action. As more units came into action the rebel advance was halted. By 23 November a stalemate had been reached, with both sides exhausted.

Miaja, a conservative but also a republican loyalist, was very close to the Spanish Communist Party (Partido Comunista Español, PCE), whose propaganda made him a symbol of Madrid's resistance to Fascism. Many later claimed that Miaja was himself a PCE member. Pedro Checa and Antonio Mije arranged with Miaja for strong representation of Communists in the council. Miaja had been instructed that membership should include all parties "in proportion to their representation in the government." The communists gained more seats in the council than they should have on that basis. The PCE and the affiliated Unified Socialist Youth (JSU: Juventudes Socialistas Unificadas) were the best organized groups with the most effective propaganda, and dominated the council. The anarcho-syndicalist CNT also readily joined the council, declaring "Long live Madrid without government!" The anti-Stalinist POUM (Partido Obrero de Unificación Marxista, Workers' Party of Marxist Unification) was excluded from the council at the insistence of the communists. The POUM paper La Batalla claimed that the Soviet ambassador had intervened to have POUM excluded.

Because most of the leading politicians had left the city, the members of the council were, as Vicente Rojo Lluch wrote, mostly "young men who had voluntarily decided to remain in the city ready to participate actively in its defense." According to a group of parliamentarians from Britain who visited Madrid early in the struggle, "The Civil Government seems to be largely in the hands of young men, sometimes barely of age. They are enthusiastic working-class lads mostly, who appear to be honestly and energetically trying to do their best." The communists held most of the important posts, and were strong in the Propaganda and Press department. The German communist Kurt Hager, alias Félix Albin, was head of the official Radio Madrid. The communists gained prestige and influence from the contribution of the International Brigades and the supply of tanks, airplanes and advisers that began to be delivered from the Soviet Union.

The council became the de facto government of Madrid. It was based in the War Ministry. Santiago Carrillo, Councillor of Public Order, appointed five of his closest associates to the key positions in his department. All were communist party members, or would join soon after. The communists soon had full control of the police. Pablo Yagüe's first action as Councillor of Supplies was to impose controls that limited shoppers to buying food at one store. If supplies there were exhausted they could try other stores in the same district. As the siege took effect, he had to deal with upward pressure on prices.