In modern German, Baden is a noun meaning "bathing" but Baden, the original name of the town, derives from an earlier plural form of Bad ("bath"). (The modern plural has become Bäder.) As with the English placename "Bath", various other Badens are at hot springs throughout Central Europe. The current doubled name arose to distinguish it from the others, particularly Baden near Vienna in Austria and Baden near Zürich in Switzerland. It is a reference to the Margraviate of Baden-Baden (1535–1771), a subdivision of the Margraviate of Baden, the territory named after the town. Baden-Baden got its formal name in 1931.
Baden-Baden lies in a valley of the Northern Black Forest in southwestern Germany. The western districts lie within the Upper Rhine Plain. The highest mountain of Baden-Baden is the Badener Höhe (), which is part of the Black Forest National Park. The old town lies on the side of a hill on the right bank of the Oos. Since the 19th century, the principal resorts have been located on the other side of the river. There are 29 natural springs in the area, varying in temperature from 46 to 67 °C (115 to 153 °F). The water is rich in salt and flows from artesian wells 1,800 m (5,900 ft) under Florentine Hill at a rate of 341 litre (90 gallons) per minute and is conveyed through pipes to the town's baths.
Roman settlement at Baden-Baden has been dated as far back as the emperor Hadrian, but on dubious authority. The known ruins of the Roman bath were rediscovered just below the New Castle in 1847 and date to the reign of Caracalla (AD 210s), who visited the area to relieve his arthritic aches. The facilities were used by the Roman garrison in Strasbourg.
The town fell into ruin but its church was first constructed in the 7th century. By 1112, it was the seat of the Margraviate of Baden. The Lichtenthal Convent (Kloster Lichtenthal) was founded in 1254. The margraves initially used Hohenbaden Castle (the Old Castle, Altes Schloss), whose ruins still occupy the summit above the town, but they completed and moved to the New Castle (Neues Schloss) in 1479. Baden suffered severely during the Thirty Years' War, particularly at the hands of the French, who plundered it in 1643. They returned to occupy the city in 1688 at the onset of the Nine Years' War, burning it to the ground the next year. The margravine Sibylla rebuilt the New Castle in 1697, but the margrave Louis William removed his seat to Rastatt in 1706. The Stiftskirche was rebuilt in 1753 and houses the tombs of several of the margraves.
The town began its recovery in the late 18th century, serving as a refuge for émigrés from the French Revolution. The town was frequented during the Second Congress of Rastatt in 1797–99 and became popular after the visit of the Prussian queen in the early 19th century. She came for medicinal reasons, as the waters were recommended for gout, rheumatism, paralysis, neuralgia, skin disorders, and stones. The Ducal government subsequently subsidized the resort's development. The town became a meeting place for celebrities, who visited the hot springs and the town's other amenities: luxury hotels, the Spielbank Casino, horse races, and the gardens of the Lichtentaler Allee. Guests included Queen Victoria, Wilhelm I, and Berlioz. The pumproom (Trinkhalle) was completed in 1842. The Grand Duchy's railway's mainline reached Baden in 1845. Reaching its zenith under Napoleon III in the 1850s and '60s, Baden became "Europe's summer capital". With a population of around 10 000, the town's size could quadruple during the tourist season, with the French, British, Russians, and Americans all well represented. (French tourism fell off following the Franco-Prussian War.)