Trier 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. 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.

With an approximate population of 105,000, Trier is the fourth-largest city in its state, after Mainz, Ludwigshafen, and Koblenz. The nearest major cities are Luxembourg (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.

Main sights include:

• the Porta Nigra, the best-preserved Roman city gate north of the Alps;
• the huge Constantine Basilica, a basilica in the original Roman sense, was the 67 m (219.82 ft) long throne hall of Roman Emperor Constantine; it is today used as a Protestant church; adjacent is the Electoral Palace, Trier;
• the Roman Trier Amphitheater;
• the 2nd century AD Roman bridge (Römerbrücke) across the Moselle, the oldest bridge north of the Alps still crossed by traffic;
• ruins of three Roman baths, among them the largest Roman baths north of the Alps; including the Barbara Baths and the Trier Imperial Baths;
• Trier Cathedral (German: Trierer Dom or Dom St. Peter), a Catholic church that dates back to Roman times; its Romanesque west façade with an extra apse and four towers is imposing and has been copied repeatedly; the Cathedral is home to the Holy Tunic, a garment said to be the robe Jesus was wearing when he died, as well as many other relics and reliquaries in the Cathedral Treasury;
• the Liebfrauenkirche (German for Church of Our Lady), which is one of the most important early Gothic churches in Germany, in some ways comparable to the architectural tradition of the French Gothic cathedrals;
• St. Matthias' Abbey (Abtei St. Matthias), a still-in-use monastery in whose medieval church the only apostle north of the Alps is held to be buried;
• St. Gangolf's church is the city's 'own' church near the main market square (as opposed to the Cathedral, the bishop's church); largely Gothic;
• Saint Paulinus' Church, one of the most important Baroque churches in Rhineland-Palatinate and designed in part by the architect Balthasar Neumann;
• two old treadwheel cranes, one being the Gothic "Old Crane" (Alte Krahnen) or "Trier Moselle Crane" (Trierer Moselkrahn) from 1413, and the other the 1774 Baroque crane called the "(Old) Customs Crane" ((Alter) Zollkran) or "Younger Moselle Crane" (Jüngerer Moselkran).
• Rheinisches Landesmuseum (an important archaeological museum for the Roman period; also some early Christian and Romanesque sculpture);
• Domschatzkammer (Treasury of Trier Cathedral; with the Egbert Shrine, the reliquary of the Holy Nail, the cup of Saint Helena and other reliquaries, liturgical objects, ivories, manuscripts, etc., many from the Middle Ages);
• Museum am Dom, formerly Bischöfliches Dom- und Diözesanmuseum (Museum of the Diocese of Trier; religious art, also some Roman artefacts); Stadtmuseum Simeonstift (history of Trier, displaying among other exhibits a scale model of the medieval city);
• Karl Marx House; a museum exhibiting Marx's personal history, volumes of poetry, original letters, and photographs with personal dedications. There is also a collection of rare first editions and international editions of his works, as well as exhibits on the development of socialism in the 19th century;
• Toy Museum of Trier;
• Ethnological and open-air museum Roscheider Hof, a museum in the neighbouring town of Konz, right at the city limits of Trier, which shows the history of rural culture in the northwest Rhineland Palatinate and in the area where Germany, Luxembourg and Lorraine meet;
• Fell Exhibition Slate Mine; site in the municipality of Fell, 20 km (12 mi) from Trier, containing an underground mine, a mine museum, and a slate mining trail.