Map - Trier (Kreisfreie Stadt Trier)

Trier (, ; Tréier ), formerly known in English as Trèves and Triers (see also names in other languages), is a city on the banks of the Moselle in Germany. It lies in a valley between low vine-covered hills of red sandstone in the west of the state of Rhineland-Palatinate, near the border with Luxembourg and within the important Moselle wine region.

Founded by the Celts in the late 4th century BC as Treuorum and conquered 300 years later by the Romans, who renamed it Augusta Treverorum ("The City of Augustus among the Treveri"), Trier is considered Germany's oldest city. It is also the oldest seat of a bishop north of the Alps. Trier was one of the four capitals of the Roman Empire during the Tetrarchy period in the late 3rd and early 4th centuries. In the Middle Ages, the archbishop-elector of Trier was an important prince of the Church who controlled land from the French border to the Rhine. The archbishop-elector of Trier also had great significance as one of the seven electors of the Holy Roman Empire. Because of its significance during the Roman and Holy Roman empires, several monuments and cathedrals within Trier are listed as a UNESCO World Heritage Site.

With an approximate population of 110,000, Trier is the fourth-largest city in its state, after Mainz, Ludwigshafen, and Koblenz. The nearest major cities are Luxembourg (50 km to the southwest), Saarbrücken (80 km southeast), and Koblenz (100 km northeast).

The University of Trier, the administration of the Trier-Saarburg district and the seat of the ADD (Aufsichts- und Dienstleistungsdirektion), which until 1999 was the borough authority of Trier, and the Academy of European Law (ERA) are all based in Trier. It is one of the five "central places" of the state of Rhineland-Palatinate. Along with Luxembourg, Metz and Saarbrücken, fellow constituent members of the QuattroPole union of cities, it is central to the greater region encompassing Saar-Lor-Lux (Saarland, Lorraine and Luxembourg), Rhineland-Palatinate, and Wallonia.

The first traces of human settlement in the area of the city show evidence of linear pottery settlements dating from the early Neolithic period. Since the last pre-Christian centuries, members of the Celtic tribe of the Treveri settled in the area of today's Trier. The city of Trier derives its name from the later Latin locative in Trēverīs for earlier Augusta Treverorum. According to the Archbishops of Trier, in the Gesta Treverorum, the founder of the city of the Trevians is Trebeta. German historian Johannes Aventinus also credited Trebeta with building settlements at Metz, Mainz, Basel, Strasbourg, Speyer and Worms.

The historical record describes the Roman Empire subduing the Treveri in the 1st century BC and establishing Augusta Treverorum about 16 BC. The name distinguished it from the empire's many other cities honoring the first Roman emperor, Augustus. The city later became the capital of the province of Belgic Gaul; after the Diocletian Reforms, it became the capital of the prefecture of the Gauls, overseeing much of the Western Roman Empire. In the 4th century, Trier was one of the largest cities in the Roman Empire with a population around 75,000 and perhaps as much as 100,000. The Porta Nigra ("Black Gate") dates from this era. A residence of the Western Roman emperor, Roman Trier was the birthplace of Saint Ambrose. Sometime between 395 and 418, probably in 407 the Roman administration moved the staff of the Praetorian Prefecture from Trier to Arles. The city continued to be inhabited but was not as prosperous as before. However, it remained the seat of a governor and had state factories for the production of ballistae and armor and woolen uniforms for the troops, clothing for the civil service, and high-quality garments for the Court. Northern Gaul was held by the Romans along a line (līmes) from north of Cologne to the coast at Boulogne through what is today southern Belgium until 460. South of this line, Roman control was firm, as evidenced by the continuing operation of the imperial arms factory at Amiens.

The Franks seized Trier from Roman administration in 459. In 870, it became part of Eastern Francia, which developed into the Holy Roman Empire. Relics of Saint Matthias brought to the city initiated widespread pilgrimages. The bishops of the city grew increasingly powerful and the Archbishopric of Trier was recognized as an electorate of the empire, one of the most powerful states of Germany. The University of Trier was founded in the city in 1473. In the 17th century, the Archbishops and Prince-Electors of Trier relocated their residences to Philippsburg Castle in Ehrenbreitstein, near Koblenz. A session of the Reichstag was held in Trier in 1512, during which the demarcation of the Imperial Circles was definitively established.

In the years from 1581 to 1593, the Trier witch trials were held. It was one of the four largest witch trials in Germany alongside the Fulda witch trials, the Würzburg witch trial, and the Bamberg witch trials, perhaps even the largest one in European history. The persecutions started in the diocese of Trier in 1581 and reached the city itself in 1587, where it was to lead to the death of about 368 people, and was as such perhaps the biggest mass execution in Europe in peacetime. This counts only those executed within the city itself. The exact number of people executed in all the witch hunts within the diocese has never been established; a total of 1,000 has been suggested but not confirmed.

In the 17th and 18th centuries, the French-Habsburg rivalry brought war to Trier. Spain and France fought over the city during the Thirty Years' War. The bishop was imprisoned by Spain and the Holy Roman Emperor for his support to France between 1635 and 1645. In later wars between the Empire and France, French troops occupied the city during the Nine Years' War, the War of the Spanish Succession, and the War of the Polish Succession. After conquering Trier again in 1794 during the French Revolutionary Wars, France annexed the city and the electoral archbishopric was dissolved. After the Napoleonic Wars ended in 1815, Trier passed to the Kingdom of Prussia. Karl Marx, the German philosopher and one of the founders of Marxism, was born in the city in 1818.

As part of the Prussian Rhineland, Trier developed economically during the 19th century. The city rose in revolt during the revolutions of 1848 in the German states, although the rebels were forced to concede. It became part of the German Empire in 1871.