The Duchy of Saxe-Lauenburg (, called Niedersachsen (Lower Saxony) between the 14th and 17th centuries), later also known as the Duchy of Lauenburg, was a reichsfrei duchy that existed 1296–1803 and 1814–1876 in the extreme southeast region of what is now Schleswig-Holstein. Its territorial center was in the modern district of Herzogtum Lauenburg and originally its eponymous capital was Lauenburg upon Elbe, though in 1619 the capital moved to Ratzeburg.