The National Democratic Union (Portuguese: União Democrática Nacional, UDN) was a political party that existed in Brazil between 1945 and 1965.

At the end of Getúlio Vargas' Estado Novo regime in 1945, political parties were allowed to reorganize themselves and to run in the general elections of that year.

UDN was defeated in the presidential elections of 1945, 1950 and 1955,[4] but remained the second largest party in the National Congress, second only to PSD, from 1945 until 1962, when it was surpassed by PTB.

However, the military regime cancelled this election and suppressed all the political parties, including UDN, during the creation of the Institutional Act Number 2, leading to the creation of the Frente Ampla (Broad Front), a short-lived and ill-fated political movement compromising of Carlos Lacerda himself and former rivals then-deposed President João Goulart and former President Juscelino Kubitschek.

The makeshift alliance aimed at the re-establishment of democratic elections in Brazil and the deposition of the recently installed Military Regime.

By then, however, many of UDN's members and Lacerda's coreligionists had already joined the National Renewal Alliance Party (Aliança Renovadora Nacional – Arena), a new party created to endorse and support the military regime, along with some members of Goulart's and Kubitschek's parties, dooming the movement to effective extinction after the creation of Institutional Act Number Five, the forced exile of many of its supporters and the establishment of the newly created Brazilian Democratic Movement (Movimento Democrático Brasileiro - MDB) as the official and legal opposition to the regime, which counted with little but existent participation of some elements in the former UDN.

[4] An expression of the attitudes of its leaders towards politics, "udenismo" was characterized by defending classical liberalism, advocating higher education and traditional morality, and repulsing populism.