A
Slavic fort of
Belizi was first mentioned in a 997 deed issued by
Emperor Otto III in favour of the
Archbishopric of Magdeburg. Whether this denotation refers to Bad Belzig or the neighbouring town of
Beelitz has not been conclusively established. Nevertheless, both towns celebrated their 1000 years anniversary in 1997. The estates however had actually already been lost in the Slavic uprising of 983 and were not conquered again until 1153 by
Albert the Bear from the
House of Ascania. In 1251 the castle
(Burg Eisenhardt) and the adjacent settlement became part of
Saxe-Wittenberg under Albert's successor
Duke Albert I. However it was claimed by the archbishops of Magdeburg, whose forces devastated Belzig in 1406. The rebuilt castle was again seized by the
Hussite general
Prokop the Great when he invaded Saxony in 1429, after which
Elector Ernest of
Saxony from the
House of Wettin enlarged it to a fortress. During the
Thirty Years' War it was seized by the troops of the
Swedish Empire in 1636, after
Elector John George I had allied with
Emperor Ferdinand II in the
Peace of Prague. The
Romanesque St Mary's Church was built in the late 13th century. According to an inscription in the
keystone of the western entrance,
Martin Luther preached here on January 14, 1530. Bad Belzig was granted
town privileges in 1702. During the
War of the Sixth Coalition on August 27, 1813, troops of the
French Empire and Saxony were attacked by
Prussian and
Russian forces near the village of Hagelberg. The encounter ended in a French defeat, while several Saxon units went over to the Prussians. According to the Final Act of the 1815
Congress of Vienna Bad Belzig was ceded to Prussia and became part of the
Province of Brandenburg, after having belonged to the Saxon Electorate for centuries. In 1934, ammunition works were established in Bad Belzig, including a
labor camp with about 1500 forced laborers. During the years 1936–1945, Burg Eisenhardt was the site of the Reichsschule (leadership school) for the Technischen Nothilfe ('technical emergency relief'.) The Technische Nothilfe was abolished in May, 1945, but the idea was revived by Otto Lummitzsch in the form of the
Technisches Hilfswerk in 1950, which exists to this day as one of the pillars of the German civil protection infrastructure. Between 1940 and 1945 a subcamp of the women's concentration camp
Ravensbrück with about 750 inmates was also located nearby. Bad Belzig was also the site of a large radio transmitter station, erected in 1939. In 1952 the town became the capital of the Belzig district and in 1993 of the newly created district of
Potsdam-Mittelmark. In 1995 Bad Belzig was awarded the official title of a climatic health resort. Effective March 2010, the town's name was changed to "Bad Belzig".
Demography File:Bevölkerungsentwicklung Bad Belzig.pdf|Development of population since 1875 within the current Boundaries: Blue line: Population. Dotted line: Comparison to population development in Brandenburg state. Grey background: Time of
Nazi Germany. Red rackground: Time of communist
East Germany File:Bevölkerungsprognosen Bad Belzig.pdf|Recent population development and projections: Population development before Census 2011 (blue line); recent population development according to the
Census in Germany in 2011 (blue bordered line); official projections for 2005-2030 (yellow line); for 2017-2030 (scarlet line); for 2020-2030 (green line) ==Politics==