| William the Victorious | |
| Duke of Brunswick-Lüneburg | |
| Noble Family: | House of Welf |
| Father: | Henry the Mild, Duke of Brunswick-Lüneburg |
| Mother: | Sophie of Pomerania |
| Spouse: | Cecilia of Brandenburg Matilda of Holstein-Schauenburg |
| Issue: | Frederick III the Turbulent William IV the Younger |
| Death Date: | 25 July 1482 (Aged 90) |
| Burial Place: | Brunswick Cathedral |
William I KG (1392 – 25 July 1482), called the Victorious (German: Wilhelm der Siegreiche), 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.
William was the eldest son of the Brunswick duke Henry the Mild from his first marriage with Sophia, daughter of the Griffin duke Wartislaw IV of Pomerania. 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. He also fought with Margrave Frederick of Meissen defeating the Hussite forces in the 1421 Battle of Brüx.
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. However, while again on a campaign in 1432, William was deposed by his brother. 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. These lands were to become known as the Principality of Calenberg (Fürstentum Calenberg), named after William's residence at Calenberg Castle and eventually raised to the Electorate of Hanover centuries later.
When the Brunswick princes of Göttingen became extinct with the death of Duke Otto the One-Eyed in 1463, William was able to take over their principality. After Henry the Peaceful died without sons in 1473, William had control of both parts of Brunswick-Wolfenbüttel again; he then ceded the smaller Calenberg and Göttingen principalities to his sons Frederick III and William the Younger.
In Berlin between 30 May/6 June 1423, William married firstly Cecilia (c. 1405 – 1449), daughter of the Hohenzollern elector Frederick I of Brandenburg. They had two sons:
In 1466, William married secondly Matilda of Holstein-Schauenburg, daughter of Count Otto II of Schauenburg-Pinneberg, and widow of Duke Bernard II of Brunswick-Lüneburg (d. 1464). Matilda died in Neustadt am Rübenberge on 22 July 1468, two days after giving birth a short-lived son, Otto (1468–1471).