William I KG, called the Victorious, a member of the House of Welf, was Duke of Brunswick-Lüneburg. He was reigning Prince of Lüneburg from 1416 to 1428 and of Brunswick-Wolfenbüttel from 1428 to 1432, counted either as William III or William IV. From 1432 he ruled over the newly established Principality of Calenberg, from 1463 also over the Principality of Göttingen. In 1473 he stepped down in favour of his sons, to assume the rule in Brunswick-Wolfenbüttel.

The House of Welf is a European dynasty that has included many German and British monarchs from the 11th to 20th century and Emperor Ivan VI of Russia in the 18th century. The originally Franconian family from the Meuse-Moselle area was closely related to the imperial family of the Carolingians.