The Danube is Europe's second longest river after the Volga. It is notable for being classified as an international waterway.

The river originates in the Black Forest in Germany as the much smaller Brigach and Breg rivers which join at the German town of Donaueschingen. After that it is known as the Danube and flows southeastward for a distance of some 2850 km (1771 miles), passing through four Central and Eastern European capitals, before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.

Known to history as one of the long-standing frontiers of the Roman Empire, the river flows through or acts as part of the borders of ten countries: Germany (7.5%), Austria (10.3%), Slovakia (5.8%), Hungary (11.7%), Croatia (4.5%), Serbia (10.3%), Bulgaria (5.2%), Moldova (1.6%), Ukraine (3.8%) and Romania (28.9%). (The percentages reflect the proportion of the total Danube basin area.)

On October 7, 2010, a toxic red sludge that burst out of a Hungarian aluminum factory's reservoir entered the Danube, threatening an ecological disaster. The pH content of the red sludge entering the Danube had been reduced to the point where it was unlikely to cause further environmental damage. It had been tested earlier at a pH level of 13 and now was down under 10, and no dead fish had been spotted where the slurry was entering the Danube.

The Danube was known in Latin as Danubius, Danuvius, Ister, in Ancient Greek as Ίστρος (Istros) . The Dacian/Thracian name was Τάναις/Donaris / Donaris (upper Danube) and Istros (lower Danube).

The name Dānuvius is presumably a loan from Celtic (Gaulish), or possibly Iranic. It is one of a number of river names derived from a Indo-European word *dānu, apparently a term for "river", but possibly also of a primeval cosmic river, and of a river goddess (see Danu (Asura)), perhaps from a root *dā "to flow/wift, rapid, violent, undisciplined." Other river names with the same etymology include Don, Donets, Dnieper and Dniestr. Dniepr and Dniestr, from Danapris and Danastius, are from Scythian *Dānu apara "posterior river" and *Dānu nazdya- "anterior river", respectively.

The Ancient Greek Istros was a borrowing from Thracian/Dacian meaning "strong, swift", akin to Sanskrit is.iras "swift".

Since the Norman conquest of England, the English language has used the French word Danube. In the languages of the modern countries through which the river flows, it is:

• Croatian: Dunav
• Bulgarian: Дунав (transliterated: Dunav)
• German: Donau
• Hungarian: Duna
• Romanian: Dunărea
• Serbian: Дунав, Dunav
• Slovak: Dunaj
• Ukrainian: Дунай (transliterated: Dunai)

In addition to the bordering countries (see above), the drainage basin includes parts of nine more countries: Italy (0.15%), Poland (0.09%), Switzerland (0.32%), the Czech Republic (2.5%), Slovenia (2.2%), Bosnia and Herzegovina (4.8%), the Republic of Macedonia, Moldova and Albania (0.03%). The highest point of the drainage basin is the summit of Piz Bernina at the Italy–Switzerland border, 4,049 metres (13,284 ft).

The Danube's watershed extends into many other countries. Many Danubian tributaries are important rivers in their own right, navigable by barges and other shallow-draught boats. From its source to its outlet into the Black Sea, its main tributaries are (in order):

1 Iller (entering at Ulm); 2 Lech; 3 Naab (entering at Regensburg); 4 Regen (entering at Regensburg); 5 Isar; 6 Inn (entering at Passau); 7 Enns; 8 Morava (entering near Devín Castle); 9 Leitha; 10 Váh (entering at Komárno); 11 Hron; 12 Ipeľ; 13 Sió; 14 Dráva; 15 Vuka (entering at Vukovar); 16 Tisza; 17 Sava (entering at Belgrade); 18 Timiş (entering at Pancevo); 19 Great Morava; 20 Caraş; 21 Jiu (entering at Bechet); 22 Iskar; 23 Olt (entering at Turnu Măgurele); 24 Osam; 24 Yantra; 25 Vedea; 26 Argeş (entering at Olteniţa); 27 Ialomiţa; 28 Siret (entering near Galaţi); 29 Prut (entering near Galaţi)

The Danube flows through the following countries and cities (ordered from the source to mouth):

• Germany
  • Donaueschingen in the State of Baden-Württemberg – Brigach and Breg rivers join to form the Danube river
  • Tuttlingen in Baden-Württemberg
  • Sigmaringen in Baden-Württemberg
  • Riedlingen in Baden-Württemberg
  • Munderkingen in Baden-Württemberg
  • Ehingen in Baden-Württemberg
  • Ulm in Baden-Württemberg
  • Neu-Ulm in Bavaria
  • Günzburg in Bavaria
  • Donauwörth in Bavaria
  • Neuburg an der Donau in Bavaria
  • Ingolstadt in Bavaria
  • Kelheim in Bavaria
  • Regensburg in Bavaria
  • Straubing in Bavaria
  • Deggendorf in Bavaria
  • Passau in Bavaria
• Austria
  • Linz, capital of Upper Austria
  • Krems on the Danube, in Lower Austria
  • Vienna – capital of Austria, where the Danube floodplain is called the Lobau, though the Innere Stadt is situated away from the main flow of the Danube (it is bounded by the Donaukanal – 'Danube canal').
• Slovakia
  • Bratislava – capital of Slovakia
  • Komárno
  • Štúrovo
• Hungary
  • Győr
  • Komárom
  • Esztergom 
  • Visegrád
  • Vác
  • Szentendre
  • Budapest – capital of Hungary
  • Százhalombatta
  • Ráckeve
  • Dunaújváros
  • Paks
  • Kalocsa
  • Baja
  • Mohács
• Croatia
  • Vukovar
  • Ilok
• Serbia
  • Apatin
  • Bačka Palanka
  • Futog
  • Veternik
  • Novi Sad
  • Sremski Karlovci
  • Zemun
  • Belgrade – the capital of Serbia
  • Pančevo
  • Smederevo
  • Veliko Gradište
  • Golubac
  • Donji Milanovac
  • Kladovo
• Bulgaria 
  • Vidin
  • Lom
  • Kozloduy
  • Oryahovo
  • Nikopol
  • Belene
  • Svishtov
  • Ruse
  • Tutrakan
  • Silistra
• Moldova
  • Giurgiuleşti
• Ukraine
  • Reni
  • Izmail
  • Kiliya
  • Vylkove
• Romania
   
  • Moldova Nouă
  • Orşova
  • Drobeta-Turnu Severin
  • Calafat
  • Bechet
  • Dăbuleni
  • Corabia
  • Turnu Măgurele
  • Zimnicea
  • Giurgiu
  • Olteniţa
  • Călăraşi
  • Feteşti
  • Cernavodă
  • Hârşova
  • Brăila
  • Galaţi – The biggest seaport on Danube
  • Isaccea
  • Tulcea
  • Sulina – the last city through which it flows

The Danube flows through four capital cities (shown in bold), more than any river in the world.

The hydrological parameters of Danube are regularly monitored in Croatia at Batina, Dalj, Vukovar and Ilok.

The Danube is navigable by ocean ships from the Black Sea to Brăila in Romania and by river ships to Kelheim, Bavaria, Germany; smaller craft can navigate further upstream to Ulm, Württemberg, Germany. About 60 of its tributaries are also navigable.

Since the completion of the German Rhine–Main–Danube Canal in 1992, the river has been part of a trans-European waterway from Rotterdam on the North Sea to Sulina on the Black Sea (3500 km). In 1994 the Danube was declared one of ten Pan-European transport corridors, routes in Central and Eastern Europe that required major investment over the following ten to fifteen years. The amount of goods transported on the Danube increased to about 100 million tons in 1987. In 1999, transport on the river was made difficult by the NATO bombing of three bridges in Serbia. The clearance of the debris was finished in 2002. The temporary pontoon bridge that hampered navigation was removed in 2005.

At the Iron Gate, the Danube flows through a gorge that forms part of the boundary between Serbia and Romania; it contains the Iron Gate I Hydroelectric Power Station dam, followed at about 60 km downstream (outside the gorge) by the Iron Gate II Hydroelectric Power Station. On 13 April 2006, a record peak discharge at Iron Gate Dam reached 15,400 m³/s.

There are three artificial waterways built on the Danube: the Danube–Tisa–Danube Canal (DTD) in the Banat and Bačka regions (Vojvodina, northern province of Serbia); the 64 km Danube–Black Sea Canal, between Cernavodă and Constanţa (Romania) finished in 1984, shortens the distance to the Black Sea by 400 km; the Rhine-Main-Danube Canal (about 171 km), finished in 1992, linking the North Sea to the Black Sea.