Luxembourg (Luxembourgish: Lëtzebuerg; French: Luxembourg; German: Luxemburg), also known as Luxembourg City (Luxembourgish: Stad Lëtzebuerg or d'Stad; French: Ville de Luxembourg; German: Stadt Luxemburg or Luxemburg-Stadt), is the capital city of Luxembourg and the country's most populous commune. Standing at the confluence of the Alzette and Pétrusse rivers in southern Luxembourg, the city lies in the center of Western Europe, situated 213 km (132 mi) by road from Brussels and 209 km (130 mi) from Cologne. The city contains Luxembourg Castle, established by the Franks in the Early Middle Ages, around which a settlement developed.

As of 31 December 2024^[update], Luxembourg City has a population of 136,208 inhabitants, which is more than three times the population of the country's second most populous commune (Esch-sur-Alzette). The population consists of 160 nationalities. Foreigners represent 70.1% of the city's population, whilst Luxembourgers represent 29.9% of the population, making it the commune with the highest proportion of foreign residents in Luxembourg.

In 2024, Luxembourg was ranked by the IMF as having the highest GDP per capita in the world at $140,310 (PPP), with the city having developed into a banking and administrative centre. In the 2019 Mercer worldwide survey of 231 cities, Luxembourg was placed first for personal safety, while it was ranked 18th for quality of living.

Luxembourg is one of the de facto capitals of the European Union (alongside Brussels, Frankfurt and Strasbourg), as it is the seat of several institutions, agencies and bodies, including the Court of Justice of the European Union, the European Court of Auditors, the Secretariat of the European Parliament, the European Public Prosecutor's Office, the European Investment Bank, the European Investment Fund, the European Stability Mechanism, Eurostat, as well as other European Commission departments and services. The Council of the European Union meets in the city for three months annually.

In the Roman era, a fortified tower guarded the crossing of two Roman roads that met at the site of Luxembourg city. Through an exchange treaty with the abbey of Saint Maximin in Trier in 963, Siegfried I of the Ardennes, a close relative of King Louis II of France and Emperor Otto the Great, acquired the feudal lands of Luxembourg. Siegfried built his castle, named Lucilinburhuc ("small castle"), on the Bock Fiels ("rock"), mentioned for the first time in the aforementioned exchange treaty.

In 987, Archbishop Egbert of Trier consecrated five altars in the Church of the Redemption (today St. Michael's Church). At a Roman road intersection near the church, a marketplace appeared around which the city developed.

The city, because of its location and natural geography, has through history been a place of strategic military significance. The first fortifications were built as early as the 10th century. By the end of the 12th century, as the city expanded westward around the new St. Nicholas Church (today the Cathedral of Notre Dame), new walls were built that included an area of 5 hectares (12 acres). In about 1340, under the reign of John the Blind, new fortifications were built that stood until 1867.

In 1443, the Burgundians under Philip the Good conquered Luxembourg. Luxembourg became part of the Burgundian, and later Spanish and Austrian empires (See Spanish Netherlands and Spanish Road) and under those Habsburg administrations Luxembourg Castle was repeatedly strengthened so that by the 16th century, Luxembourg itself was one of the strongest fortifications in Europe. Subsequently, the Burgundians, the Spanish, the French, the Spanish again, the Austrians, the French again, and the Prussians conquered Luxembourg.^{[citation needed]}