The Habsburg monarchy, also known as Habsburg Empire, or Habsburg Realm (/ˈhæpsbɜːrɡ/), was the collection of empires, kingdoms, duchies, counties and other polities (composite monarchy) that were ruled by the House of Habsburg. From the 18th century it is also referred to as the Austrian monarchy, the Austrian Empire (Latin: Monarchia Austriaca) or the Danubian monarchy.

The history of the Habsburg monarchy can be traced back to the election of Rudolf I as King of Germany in 1273 and his acquisition of the Duchy of Austria for the Habsburgs in 1282. In 1482, Maximilian I acquired the Netherlands through marriage. Both realms passed to his grandson and successor, Charles V, who also inherited the Spanish throne and its colonial possessions, and thus came to rule the Habsburg empire at its greatest territorial extent. The abdication of Charles V in 1556 led to a division within the dynasty between his son Philip II of Spain and his brother Ferdinand I, who had served as his lieutenant and the elected king of Hungary, Croatia and Bohemia. The Spanish branch (which held all of Iberia, the Netherlands, and lands in Italy) became extinct in the male line in 1700, but continued through the female line through the House of Bourbon. The Austrian branch (which ruled the Holy Roman Empire, Hungary, Bohemia and various other lands) was itself split into different branches in 1564 but reunited 101 years later. It became extinct in the male line in 1740, but continued through the female line as the House of Habsburg-Lorraine.

The Habsburg monarchy was a union of crowns, with only partial shared laws and institutions other than the Habsburg court itself; the provinces were divided in three groups: the Archduchy proper, Inner Austria that included Styria and Carniola, and Further Austria with Tyrol and the Swabian lands. The territorial possessions of the monarchy were thus united only by virtue of a common monarch. The Habsburg realms were unified in 1804 with the formation of the Austrian Empire and later split in two with the Austro-Hungarian Compromise of 1867. The monarchy began to fracture in the face of inevitable defeat during the final years of World War I and ultimately disbanded with the proclamation of the Republic of German-Austria and the First Hungarian Republic in late 1918.

In historiography, the terms "Austria" or "Austrians" are frequently used as shorthand for the Habsburg monarchy since the 18th century. From 1438 to 1806, the rulers of the House of Habsburg almost continuously reigned as Holy Roman Emperors. However, the realms of the Holy Roman Empire were mostly self-governing and are thus not considered to have been part of the Habsburg monarchy. Hence, the Habsburg monarchy (of the Austrian branch) is often called "Austria" by metonymy. Around 1700, the Latin term monarchia austriaca came into use as a term of convenience. Within the empire alone, the vast possessions included the original Hereditary Lands, the Erblande, from before 1526; the Lands of the Bohemian Crown; the formerly Spanish Austrian Netherlands from 1714 until 1794; and some fiefs in Imperial Italy. Outside the empire, they encompassed all the Kingdom of Hungary as well as conquests made at the expense of the Ottoman Empire. The dynastic capital was Vienna, except from 1583 to 1611, when it was in Prague.

The first Habsburg who can be reliably traced was Radbot of Klettgau, who was born in the late 10th century; the family name originated with Habsburg Castle, in present-day Switzerland, which was built by Radbot. After 1279, the Habsburgs came to rule in the Duchy of Austria, which was part of the elective Kingdom of Germany within the Holy Roman Empire. King Rudolf I of Germany of the Habsburg family assigned the Duchy of Austria to his sons at the Diet of Augsburg (1282), thus establishing the "Austrian hereditary lands". From that moment, the Habsburg dynasty was also known as the House of Austria. Between 1438 and 1806, with few exceptions, the Habsburg Archduke of Austria was elected as Holy Roman Emperor.

The Habsburgs grew to European prominence as a result of the dynastic policy pursued by Maximilian I, Holy Roman Emperor. Maximilian married Mary of Burgundy, thus bringing the Burgundian Netherlands into the Habsburg possessions. Their son, Philip the Handsome, married Joanna the Mad of Spain (daughter of Ferdinand II of Aragon and Isabella I of Castile). Charles V, Holy Roman Emperor, the son of Philip and Joanna, inherited the Habsburg Netherlands in 1506, Habsburg Spain and its territories in 1516, and Habsburg Austria in 1519.

At this point, the Habsburg possessions were so vast that Charles V was constantly travelling throughout his dominions and therefore needed deputies and regents, such as Isabella of Portugal in Spain and Margaret of Austria in the Low Countries, to govern his various realms. At the Diet of Worms in 1521, Emperor Charles V came to terms with his younger brother Ferdinand. According to the Habsburg compact of Worms (1521), confirmed a year later in Brussels, Ferdinand was made Archduke, as a regent of Charles V in the Austrian hereditary lands.

Following the death of Louis II of Hungary in the Battle of Mohács against the Ottoman Turks, Archduke Ferdinand (who was his brother-in-law by virtue of an adoption treaty signed by Maximilian and Vladislaus II, Louis's father at the First Congress of Vienna) was also elected the next king of Bohemia and Hungary in 1526. Bohemia and Hungary became hereditary Habsburg domains only in the 17th century: Following victory in the Battle of White Mountain (1620) over the Bohemian rebels, Ferdinand II promulgated a Renewed Land Ordinance (1627/1628) that established hereditary succession over Bohemia. Following the Battle of Mohács (1687), in which Leopold I reconquered almost all of Ottoman Hungary from the Turks, the emperor held a diet in Pressburg to establish hereditary succession in the Hungarian kingdom.