Charles I or Karl I (German: Karl Franz Josef Ludwig Hubert Georg Otto Maria, Hungarian: Károly Ferenc József Lajos Hubert György Ottó Mária; 17 August 1887 – 1 April 1922) was Emperor of Austria, King of Hungary (as Charles IV, Hungarian: IV. Károly), King of Croatia, King of Bohemia (as Charles III, Czech: Karel III.), and the last of the monarchs belonging to the House of Habsburg-Lorraine to rule over Austria-Hungary. The son of Archduke Otto of Austria and Princess Maria Josepha of Saxony, Charles became heir presumptive of Emperor Franz Joseph after his uncle Archduke Franz Ferdinand of Austria was assassinated in 1914. In 1911, he married Princess Zita of Bourbon-Parma. He is venerated in the Catholic Church, having been Beatified by Pope John Paul II on 3 October 2004, and is known to the Catholic Church as Blessed Karl of Austria.Charles succeeded to the thrones in November 1916 following the death of his grand-uncle, Franz Joseph. He made secret attempts to negotiate Austria-Hungary's exit from the First World War but was unsuccessful. Despite Charles's efforts to preserve the empire by transforming it into a federal union, Austria-Hungary hurtled towards disintegration: Czechoslovakia and the State of Slovenes, Croats and Serbs were proclaimed, and Hungary broke monarchic ties to Austria by the end of October 1918. Following the Armistice of 11 November 1918, Charles "renounced participation" in state affairs, but did not abdicate. The Republic of German-Austria was proclaimed the following day, and in April 1919 he was formally dethroned by the Austrian Parliament and exiled to Switzerland.

Charles spent the remaining years of his life attempting to restore the monarchy. He made two attempts to reclaim the Hungarian throne in 1921; both failed due to a lack of support from Hungary's regent Miklós Horthy. Charles was exiled for a second time to the Portuguese island of Madeira, where he soon fell ill and died of respiratory failure in 1922.