These villages became districts (Ortsteile) of Belzig: The forest of Verlorenwasser near Werbig encompassed the geographical centre of East Germany.

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.

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.

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.

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.

During the years 1936–1945, Burg Eisenhardt was the site of the Reichsschule (leadership school) for the Technischen Nothilfe ('technical emergency relief').

The 2014 elections showed the following results: The town's shield depicts the coat of arms of Saxony due to the long-time affiliation with the Saxon Electorate.

In 1989, Bad Belzig launched an ambitious programme to become a spa town (achieving recognition in 2009) and promote tourism.