Saxe-Hildburghausen (German: Sachsen-Hildburghausen) was an Ernestine duchy and Imperial Estate of the Holy Roman Empire in the southern side of the present State of Thuringia in Germany.
After the duke of Saxe-Gotha, Ernest the Pious, died on 26 March 1675 in Gotha, the duchy was divided on 24 February 1680 among his seven surviving sons.
When Albert V, the duke of Saxe-Coburg, died in 1699 without any surviving descendants, disputes arose over the inheritance but, eventually, in 1714, Saxe-Hildburghausen agreed to exchange the district of Schalkau for parts of the other Saxon duchies – a piece of the former Duchy of Saxe-Römhild, the District of Behrungen, including the winery, and the monastery estate of Milz as well as the former properties of the Echter family of Mespelbrunn.
However, the elaborate buildings and courtyards of the princes strained the finances of the duchy so much that, in 1769, a forced management of debts by an Imperial Debit commission had to be ordered.
On 12 November 1826 the decision, from the arbitration of the supreme head of the family, King Frederick Augustus I of Saxony, resulted in the extensive rearrangement of the Ernestine duchies.