The Duchy of Saxe-Merseburg was a territory of the Holy Roman Empire, with Merseburg as its capital. It existed from 1656/57 to 1738 and was owned by the Albertine branch of the House of Wettin.
The Saxon Elector John George I stipulated in his will dated 20 July 1652 that his three younger sons should receive secundogeniture principalities. After the elector died on 8 October 1656, his sons concluded the "friend-brotherly main treaty" in Dresden on 22 April 1657 and a further treaty in 1663 delineating their territories and sovereign rights definitively. The treaties created three duchies:
Prince Christian, the third oldest son, received, among other properties, the castles, cities and districts of Merseburg, Plagwitz, Rückmarsdorf, Delitzsch (with Delitzsch Castle), Bad Lauchstädt, Schkeuditz, Lützen, Bitterfeld, Zörbig, the County of Brehna and the Margraviate of Lower Lusatia, including the cities and castles of Lübben, Doberlug, Finsterwalde, Döbern, Forst and Guben. Many of these territories had belonged to the Diocese of Merseburg until it was secularized in 1562.
The area of Saxe-Merseburg stretched to the city limits of Leipzig. The customs station was in what is now the inner city district of Lindenau.
After the death of the last male heir of the Saxon branch line in 1738, the Duchy of Saxe-Merseburg fell back to the Electorate of Saxony.
To supply his three younger sons with incomes befitting a duke, Duke Christian I created apanages for his younger sons during his lifetime. These territories remained dependent on the main line and their soverignty was severely restricted. They were named after their owner's residences and disappeared with the death of their first duke, because none of them fathered a male heir. Before it died out, the Saxe-Merseburg-Spremberg line inherited all of Saxe-Merseburg.
- Until 1715 August (born: 15 February 1655 in Merseburg; died: 27 March 1715 in Zörbig), Duke of Saxe-Merseburg-Zörbig
- Until 1690 Philipp (born: 26 October 1657 in Merseburg; died: 1 July 1690 in Fleurus), Duke of Saxe-Merseburg-Lauchstädt
- Until 1731 Heinrich (born: 2 September 1661 in Merseburg; died: 28 July 1738 in Doberlug), Duke of Saxe-Merseburg-Spremberg until 1731, inherited Saxe-Merseburg in 1731