William I KG (c. 1392 – 25 July 1482), called the Victorious (German: Wilhelm der Siegreiche), a member of the House of Welf, was Duke of Brunswick-Lüneburg.

Upon his father's death in 1416, he inherited the Brunswick Principality of Lüneburg which he ruled jointly with his younger half-brother Henry the Peaceful.

William turned out to be an energetic ruler; he soon entered into numerous feuds with neighbouring princes such as the Archbishop of Bremen and the Bishop of Hildesheim, and supported the Counts of Schauenburg and Holstein in their fight against King Eric of Denmark.

In 1428, William and Henry swapped Lüneburg with their uncle Duke Bernard I, for the Principality of Brunswick-Wolfenbüttel and the later Calenberg territory stretching from the Deister hill range to the Leine river.

After fierce fratricidal warfare, Henry retained Wolfenbüttel while William was compensated with the territory west of the River Leine, separated from the rest of the duchy by the Prince-Bishopric of Hildesheim.