Frederick III (German: Friedrich III, 21 September 1415 – 19 August 1493) was Holy Roman Emperor from 1452 until his death in 1493. He was the penultimate emperor to be crowned by the Pope, and the last to be crowned in Rome. He was the fourth King of the Romans and the first Holy Roman Emperor from the House of Habsburg, which was to retain the title with one gap until it was declared at an end by Emperor Francis II, in 1806.

Prior to his imperial coronation, he was duke of the Inner Austrian lands of Styria, Carinthia and Carniola from 1424, and also acted as regent over the Duchy of Austria from 1439. He was elected and crowned King of Germany in 1440. His reign of 53 years is the longest in the history of the Holy Roman Empire or the German monarchy. Upon his death in 1493 he was succeeded by his son Maximilian.

During his reign, Frederick concentrated on re-uniting the Habsburg "hereditary lands" of Austria and took a lesser interest in Imperial affairs. Nevertheless, by his dynastic entitlement to Hungary as well as by the Burgundian inheritance, he laid the foundations for the later Habsburg Empire. Despite being mocked as "Arch-Sleepyhead of the Holy Roman Empire" (German: Erzschlafmütze) during his lifetime, he is today increasingly seen as an effective ruler.

Historian Thomas A. Brady Jr. credited Frederick with leaving a credible claim on the imperial title and a secure grip on the Austrian lands, now organized as a single state, for his son. This imperial revival (as well as the rise of the territorial state) began under the reign of Frederick.

Born at the Tyrolean residence of Innsbruck in 1415, Frederick was the eldest son of the Inner Austrian duke Ernest the Iron, a member of the Leopoldian line of the Habsburg dynasty, and his second wife Cymburgis of Masovia. According to the 1379 Treaty of Neuberg, the Leopoldinian branch ruled over the duchies of Styria, Carinthia and Carniola, or what was referred to as Inner Austria. Only three of Frederick's eight siblings survived childhood: his younger brother Albert (later to be Albert VI, archduke of Austria), and his sisters Margaret (later the electress of Saxony) and Catherine. In 1424, nine-year-old Frederick's father died, making Frederick the duke of Inner Austria, as Frederick V, with his uncle, Duke Frederick IV of Tyrol, acting as regent.

From 1431, Frederick tried to obtain majority (to be declared "of age", and thus allowed to rule) but for several years was denied by his relatives. Finally, in 1435, Albert V, duke of Austria (later Albert II, the king of Germany), awarded him the rule over his Inner Austrian heritage. Almost from the beginning, Frederick's younger brother Albert asserted his rights as a co-ruler, as the beginning of a long rivalry. Already in these years, Frederick had begun to use the symbolic A.E.I.O.U. signature as a kind of motto with various meanings. In 1436 he made a pilgrimage to the Holy Land, accompanied by numerous nobles knighted by the Order of the Holy Sepulchre, which earned him great reputation.

Upon the death of his uncle Duke Frederick IV in 1439, Frederick took over the regency of Tyrol and Further Austria for the duke's heir Sigismund. Again he had to ward off the claims raised by his brother Albert VI; he prevailed by the support of the Tyrolean aristocracy. Likewise he acted as regent for his nephew Ladislaus the Posthumous, son of late King Albert II and his consort Elizabeth of Luxembourg, in the duchy of Austria (Further Austria). (Ladislaus would die before coming of age). Frederick was now the undisputed head of the Habsburg dynasty, though his regency in the lands of the Albertinian Line (Further Austria) was still viewed with suspicion.

As a cousin of late King Albert II, Frederick became a candidate for the 1440 imperial election. On 2 February 1440, the prince-electors convened at Frankfurt and unanimously elected him King of the Romans as Frederick IV; his rule was still based on his hereditary lands of Styria, Carinthia and Carniola, or Inner Austria.