Altona (German: [ˈaltonaː] ^ⓘ), also called Hamburg-Altona, is the westernmost urban borough (Bezirk) of the German city state of Hamburg. Located on the right bank of the Elbe river, Altona had a population of 270,263 in 2016.

From 1640 to 1864, Altona was under the administration of the Danish monarchy. Altona was an independent borough until 1937.

The first documented mention of Altona was in 1537 as a tavern "Lohekrug" for the fishermen. For the competing Hamburg innkeepers, the Lohekrug was “all to na” (all too near) to the border with Hamburg, from which the name Altona may have been derived. Altona grew up around the inn as a settlement of fishermen and craftsmen, which was promoted by the rulers of the land, the Counts of Holstein-Pinneberg, in the spirit of mercantilism. In 1640, Altona was part of Holstein-Glückstadt, and in 1664 the place was granted municipal rights by the king Frederik III of Denmark, who then ruled in personal union as Duke of Holstein in the Holy Roman Empire. Altona was one of his monarchy's most important harbor towns. The railway from Altona to Kiel, the Hamburg-Altona–Kiel railway (Danish: Christian VIII Østersø Jernbane), was opened in 1844.

The wars between Denmark and the German Confederation – the First Schleswig War (1848–1851) and the Second Schleswig War (February–October 1864) led to Denmark's cession of the Duchies of Schleswig, Holstein and Lauenburg; they were initially jointly administered as a condominium by Prussian and Austrian administration. With the Gastein Convention of 14 August 1865, Holstein came under solely Austrian administration, while Schleswig and Lauenburg came under Prussian authority. After the Austro-Prussian War, Schleswig-Holstein as a whole became the Prussian province of Schleswig-Holstein in 1867 and as such Altona became part of the German Empire in 1871. In the same year, the town was hit by cholera, with a minimum of 16 casualties in Altona.

Because of severe restrictions on the number of Jews allowed to live in Hamburg until 1864 (with the exception of 1811–1815), a major Jewish community developed in Altona starting in 1611, when Count Ernest of Schaumburg and Holstein-Pinneberg granted the first permanent residence permits to Ashkenazic Jews. Members did business both in Hamburg and in Altona itself. All that remains after the Nazi Holocaust during World War II are the Jewish cemeteries, but in the 17th, 18th, and 19th centuries, the community was a major center of Jewish life and scholarship. Holstein-Pinneberg, and later Danish Holstein, imposed lower taxes and fewer civil restrictions on their Jewish communities than the government of Hamburg.

During the Weimar era following World War I, the town of Altona experienced significant labor strikes and street unrest amid widespread economic instability. Inflation in Germany was a major issue, and in 1923, Altona's mayor, Max Brauer, addressed the problem by partially paying town personnel with gas meter tokens, which retained their value despite the inflation.

The most notable event at that time was the Altona Bloody Sunday (German: Altonaer Blutsonntag) on 17 July 1932, when 18 people were killed, all but two by police, during a violent clash between Nazi marchers and members or supporters of the Communist Party. After police raids and a special court, on 1 August 1933, Bruno Tesch and others were found guilty and put to death by beheading with a hand-held axe. In 1938, the Greater Hamburg Act removed Altona from the Free State of Prussia in 1937 and merged it (and several surrounding towns) with the Free and Hanseatic City of Hamburg.

In the 1990s, the Federal Republic of Germany reversed the convictions of Tesch and the other men who were put to death, clearing their names.