William I, Duke of Brunswick-Lüneburg

William I (1270–1292) was the third son of Duke Albert I of Brunswick and Adelaide of Montferrat. In the 1286 partition after their father's death, William received Brunswick itself and its surrounding lands, while Henry and Albert took Grubenhagen and Göttingen; open war between the brothers followed in 1288.

He married Elisabeth, daughter of Landgrave Henry I of Hesse, probably without children. William died at Brunswick in 1292, his lands passing to Albert after an inheritance dispute with Henry.