Regensburg, one of the oldest German cities, is a cultural center with many historic monuments. Dating back, as Radasbona, to Celtic times, it was an important Roman frontier station, known as Castra Regina. An abbey was founded there in the mid-7th cent., and St. Boniface established an episcopal see in 739. Regensburg was captured (788) by Charlemagne when he subjugated Bavaria. The city was one of the most prosperous commercial centers of medieval Germany, trading especially with India and the Middle East. In 1245, Regensburg was made a free imperial city; part of the adjacent countryside, however, remained in ecclesiastical hands.

The city proper accepted the Reformation in the 16th cent., but soon thereafter it was strongly influenced by the Roman Catholic Counter Reformation (late 16th cent.). Its commerce declined in the 15th and 16th cent., as a result of the shifting of international trade routes. In the Thirty Years War, Regensburg, garrisoned by Bavarian troops, was bombarded and captured (1633) by the Protestant general Bernhard of Saxe-Weimar, but it was recovered (1634) by imperial forces under Ferdinand of Hungary and Bohemia (later Emperor Ferdinand III).

Regensburg was frequently the meeting place of the imperial diet from 1532, and from 1663 to 1806 it was the permanent seat of the diet. The diet that met there from 1801 to 1803 under the influence of Napoleon Bonaparte completely reorganized the moribund Holy Roman Empire. The city and the bishopric of Regensburg (later raised to an archbishopric) were given, with Aschaffenburg, to K. T. von Dalberg. In 1810 the city passed to Bavaria and became the capital of the Upper Palatinate. Regensburg was bombed extensively by the Allies in World War II, largely because it was an airplane-manufacturing center; most of its medieval buildings survived with surprisingly little damage.

Noteworthy structures of the city include the Gothic cathedral (13th–16th cent.); parts of the Porta Praetoria, a Roman gate (built A.D. 179); the Schottenkirche St. Jakob, a 12th-century church; an 11th-century chapel (with later decoration in the rococo style); the old city hall (14th–18th cent.), where the imperial diet met; and St. Emmeram, the episcopal residence (a former Benedictine convent founded in the 7th cent.). The church of the Benedictine convent, with foundations dating from the 8th cent. to the 12th cent. and with an 18th-century baroque interior, contains the tombs of Emperor Arnulf and of Louis the Child. Regensburg is the seat of a university (founded 1965) and schools of engineering and church music. The city was a residence of the painter Albrecht Altdorfer and the astronomer Johannes Kepler, both of whom died there.