The Duchy of Saxe-Lauenburg (German: Herzogtum Sachsen-Lauenburg, called Niedersachsen (Lower Saxony) between the fourteenth and seventeenth centuries), later also known as the Duchy of Lauenburg, is a reichsfrei duchy that exists 1296–1803 and 1814–1876 in the extreme southeast region of what is now Schleswig-Holstein.

Its territorial center is in the modern district of Herzogtum Lauenburg and originally its eponymous capital is Lauenburg upon Elbe, though in 1619 the capital moves to Ratzeburg.