The city adopted the Protestant Reformation in 1542 and its Town Council remained entirely Lutheran.
From 1663 to 1806, the city was the permanent seat of the Imperial Diet of the Holy Roman Empire, which became known as the Perpetual Diet of Regensburg.
The Celtic name Radasbona was the oldest given to a settlement near the present city. In 179, a new Roman fort Castra Regina ("fortress by the river Regen") was built for Legio III Italica during the reign of Emperor Marcus Aurelius.
On 8 December 899 Arnulf of Carinthia, descendant of Charlemagne, died at Regensburg (known as Ratisbon at the time), Bavaria, Germany.Between 11, the Stone Bridge across the Danube was built at Regensburg.In 1810 Dalberg ceded Regensburg to the Kingdom of Bavaria, he himself being compensated by the award of Fulda and Hanau to him under the title of "Grand Duke of Frankfurt".Between April 19 and April 23, 1809, Regensburg was the scene of the Battle of Ratisbon between forces commanded by Baron de Coutaud (the 65th Ligne) and retreating Austrian forces.It is believed that as early as in late Roman times the city was the seat of a bishop, and St Boniface re-established the Bishopric of Regensburg in 739.
From the early 6th century, Regensburg was the seat of a ruling family known as the Agilolfings.Thus, Regensburg was one of the central towns of the Empire, attracting visitors in large numbers. But the town of Regensburg must not be confused with the Bishopric of Regensburg.Although the Imperial city had adopted the Reformation, the town remained the seat of a Roman Catholic bishop and several abbeys. Emmeram, Niedermünster and Obermünster, were estates of their own within the Holy Roman Empire, meaning that they were granted a seat and a vote at the Imperial Diet ().In 792, Regensburg hosted the ecclesiastical section of Charlemagne's General Assembly, the bishops in council who condemned the heresy of adoptionism taught by their Spanish counterparts, Elipandus of Toledo and Felix of Urgel.After the partition of the Carolingian Empire in 843, the city became the seat of the Eastern Frankish ruler, Louis II the German.In 1245 Regensburg became a Free Imperial City and was a trade centre before the shifting of trade routes in the late Middle Ages.