Ferdinand I (Spanish: Fernando I) (10 March 1503 – 25 July 1564) was Holy Roman Emperor from 1558, king of Bohemia and Hungary from 1526, and king of Croatia from 1527 until his death. Before his accession, he ruled the Austrian hereditary lands of the Habsburgs in the name of his elder brother, Charles V, Holy Roman Emperor. Also, he often served as Charles' representative in Germany and developed useful relationships with German princes.

The key events during his reign were the contest with the Ottoman Empire, whose great advance into Central Europe began in the 1520s, and the Protestant Reformation, which resulted in several wars of religion. Ferdinand was able to defend his realm and make it somewhat more cohesive, but he could not conquer the major part of Hungary. His flexible approach to Imperial problems, mainly religious, finally brought more result than the more confrontational attitude of his brother.

Ferdinand's motto was Fiat iustitia, et pereat mundus: "Let justice be done, though the world perish".

Ferdinand was born in Alcalá de Henares, Spain, the son of Queen Joanna I of Castile from the House of Trastámara (herself the daughter of the Catholic Monarchs Isabel I of Castile and Ferdinand of Aragon) and Habsburg Archduke Philip the Handsome, who was heir to Maximilian I, Holy Roman Emperor. Ferdinand shared his customs, culture, and even his birthday with his maternal grandfather Ferdinand II of Aragon. He was born, raised, and educated in Spain, and did not learn German when he was young.

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