Trier ( TREER, German: [tʁiːɐ̯] (listen); Luxembourgish: Tréier [ˈtʀəɪɐ] (listen)), formerly known in English as Trèves ( TREV;) 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 City (50 km or 31 mi to the southwest), Saarbrücken (80 kilometres or 50 miles southeast), and Koblenz (100 km or 62 mi 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.