William I, Duke of Brunswick-Lüneburg

William (c. 1270 – 30 September 1292, in Brunswick), Duke of Brunswick-Lüneburg, briefly ruled part of the duchy.

William was the third son of Albert I, Duke of Brunswick-Lüneburg. On Albert's death on 1279, the three eldest brothers succeeded him, but were put under guardianship of Conrad, Prince-Bishop of Verden. After they reached majority, they divided the territory among each other in 1291: William received the northern part of their father's state, including Brunswick, Schöningen, the Harzburg, Seesen, and Königslutter. The brothers disagreed over control of the City of Brunswick.

Only a year later, William died, and his brothers divided his territory among each other. He had married Elizabeth, daughter of Henry I, Landgrave of Hesse, but the marriage apparently remained childless.