Otto the Mild (24 June 1292 – 30 August 1344), Duke of Brunswick-Lüneburg, ruled over the Brunswick part of the duchy.
Otto was the eldest son of Albert II, Duke of Brunswick-Lüneburg. Otto and his brothers succeeded on their father's death in 1318; he served as his brothers' guardian while they were not of age. In 1323, he acquired the Altmark from Brandenburg as an inheritance of his wife's; but he sold it in 1343, when he failed to establish control there.
Otto died in 1344 in Göttingen and was succeeded by his brothers.