The Duchy of Brunswick-Lüneburg, or more properly Duchy of Brunswick and Lüneburg, is an historical ducal state from the late Middle Ages until the late Early Modern era within the North-Western domains of the Holy Roman Empire of the German Nation, in what is now northern Germany.

The dukedom emerged in 1235 from the allodial lands of the House of Welf in Saxony and was granted as an imperial fief to Otto the Child, a grandson of Henry the Lion.

Its name comes from the two largest towns in the territory: Brunswick and Lüneburg.

The duchy is divided several times during the High Middle Ages among various lines of the House of Welf, but the rulers all continue to be styled as the "Duke of Brunswick-Lüneburg" in addition to their particular title as "Prince of Lüneburg" or "Prince of Wolfenbüttel", etc.

The individual principalities making up the duchy continue to exist until the end of the Holy Roman Empire in 1806.

Following the Congress of Vienna in 1814/15, the territories become part of the Kingdom of Hanover and Duchy of Brunswick.