John of Austria (24 February 1547 – 1 October 1578), in English traditionally known as Don John of Austria, in Spanish as Don Juan de Austria and in German as Ritter Johann von Österreich, was an illegitimate son of Holy Roman Emperor Charles V. He became a military leader in the service of his half-brother, King Philip II of Spain and is best known for his naval victory at the Battle of Lepanto in 1571 against the Ottoman Empire.