Ferdinand I, Holy Roman Emperor (March 10, 1503 - July 27, 1564) was one of the Habsburg emperors that at various periods during his life ruled over Austria, Germany, Bohemia, and Hungary.

Ferdinand was born in Madrid, the son of Philip von Habsburg (1478-1506) and his wife Juana of Castile (1479-1555).

Ferdinand was the younger brother of Charles who became the Emperor, and was instead given control of the Habsburg Hereditary Lands (roughly modern-day Austria). On May 25, 1521 in Linz, Austria, he married Anne of Bohemia (1503-1547), daughter of King Ladislaus V of Hungary and his wife Anne de Foix. He generally ruled the Empire during his brother's absence, and in 1531 was elected King of the Romans, making him his brother's designated heir.

After Suleiman the Magnificent defeated Louis II the king of Bohemia and Hungary and Ferdinand's brother-in-law on August 29, 1526, Ferdinand was elected king of Bohemia on October 24, while the Kingdom of Hungary became subject to a dynastic dispute between the Habsburgs and the Zapolyas, who were headed by John Zápolya, prince of Transylvania. Each was supported by a certain amount of the nobility from the Hungarian kingdom, while Ferdinand also had the support of his brother Charles.

In 1529, his forces successfully repelled Suleiman's assault on his capital at Vienna (First Battle of Vienna), although Ferdinand had notably already fled for Bohemia. Finally, in 1533 Ferdinand signed a peace treaty with the Ottoman Empire, splitting the Kingdom of Hungary between the Habsburg part in the west and John Zápolya's domain in the east.

Starting with Ferdinand in 1527, Habsburgs would eventually convert the elected crowns of both Bohemia and Hungary into their hereditary possession. In 1538, by the Treaty of Nagyvárad/Grosswardein, Ferdinand became Zápolya's successor, but was unable to enforce this agreement during his lifetime.

After the abdication of his brother, Charles V, Ferdinand ruled the European section of the Holy Roman Empire (Austria, Bohemia and parts of Germany) as emperor between 1556 and 1564. Charles also agreed to exclude his son Philip from the German succession, which instead passed to Ferdinand's eldest son Maximilian (1527-1576).

Other children of Ferdinand and Anne included:

• Marie (1531-1581)
• Johanna (1547-1578)
• Anna (1528-~1590)
• Karl II (1540-1590)
• Elisabeth
• Catharine

He died in Vienna and is buried in St. Vitus Cathedral in Prague.

Names in other languages: German/Czech/Slovak/Croatian: Ferdinand I., Hungarian: I. Ferdinánd