The First Brazilian Republic, also referred to as the Old Republic (Portuguese: República Velha, Portuguese pronunciation: [ʁeˈpublikɐ ˈvɛʎɐ]), officially the Republic of the United States of Brazil, was the Brazilian state in the period from 1889 to 1930. The Old Republic began with the coup d'état that deposed emperor Pedro II in 1889, and ended with the Revolution of 1930 that installed Getúlio Vargas as a new president. During the First Republic, the country's presidency was dominated by the most powerful states of São Paulo and Minas Gerais. Because of the power of these two states, based on the production of coffee and dairy, respectively, the Old Republic's political system has been described as "milk coffee politics". At local level, the country was dominated by a form of machine politics known as coronelism, in which the political and economic spheres were centered around local bosses, who controlled elections and would often conduct electoral fraud.

The country was also marked by a series of rebellions and revolutions against the ruling oligarchies, which culminated into the Revolution of 1930, when the Liberal Alliance, a force of urban middle-class, planters from outside São Paulo and military reformists composed mostly by junior officers (known as Tenetism), deposed ruling president Washington Luís (representative of the São Paulo oligarchies) and led to the ascension of Getúlio Vargas as president, heralding the start of the Vargas Era.

After the Paraguayan War, the Brazilian military gained political influence. Emperor Pedro II's worsening health gradually removed him from his active role in politics, which caused tension between the imperial elite and the military as there was no moderating force between them. Pedro II's successor, Princess Isabel, had a husband who had a controversial personality. The church had also worn down the government to some extent.

In 1889, Deodoro da Fonseca, a marshal of the Brazilian Army, attended a meeting with prestigious civilian and military men who pressured him to begin a movement against the monarchy; there had been a rumor that he would be arrested. This resulted in him launching a coup d'état on 15 November 1889, which was bloodless and unopposed. It is unknown if whether on the day itself he had declared the birth of the republic, or had only removed the Prime Minister, the Viscount of Ouro Preto from power.

The officers who joined Field Marshal Deodoro da Fonseca in ending the Empire had made an oath to uphold it. The officer corps would eventually resolve the contradiction by linking its duty to Brazil itself, rather than to transitory governments. The Republic was born rather accidentally: Deodoro had intended only to replace the cabinet, but the republicans manipulated him into founding a republic.

Following the coup, Deodoro da Fonseca was chosen to be the leader of the new republic.

Many of the tasks which the new government were faced with were solved by executive decree, which drew opposition from republicans and non-republicans. The governments' first decree stated that until a constitution was adopted, a federal republic would govern. Followed by decrees which allowed those with literacy and were above the age of 21 to vote, (it was implicitly understood that women did not vote), separated church and state, and dissolved state assemblies under the Empire.

Ruy Barbosa was the main handler of financial problems. In the last years of the imperial regime, many financial problems had begun. Ruy Barbosa attempted to solve these problems by allowing certain banks and businesses concerns favored by the government to triple the nation's credit. This plan was called the "Encilhamento". The immediate effects were mass speculation and inflation, which caused foreign investors and banks to pull out of Brazil. It would be several years before the country regained the trust of foreign bankers.