Otto III, duke of Brunswick-Lüneburg

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Otto III, Herzog von Braunschweig-Lüneburg, was born about 1296, the second son of Otto II 'der Strenge', Herzog von Braunschweig-Lüneburg, and Matilde von Bayern. On 22 March 1310 he married Matilde von Mecklenburg, daughter of Heinrich II 'the Lion', Fürst zu Mecklenburg, and Beatrix von Brandenburg, heiress of Stargard. Of their three children only Matilde would have progeny, marrying Otto II, Graf von Waldeck. His only son Otto had drowned in the river Ilmenau.

In 1314 Otto was brought into aspects of the rule of the duchy by his father. After his father's death in 1330 he and his brother Wilhelm decided to ignore their father's wish that they should divide the duchy, and they ruled it together. The main focus of their rule in the first years was the territorial consolidation of the duchy. Through the acquisition of the village of Fallersleben and the counties of Papenheim and Wettmarshagen, they greatly increased their holdings in the area of Gifhorn. Another main focus was political support for the towns, which were growing in economic importance. Lüneburg's trade profited in particular from making the river Ilmenau navigable by ships between Lüneburg and Uelzen, as well as from economic contracts between the Lüneburg dukes and the dukes of Sachsen-Lauenburg.

Otto III died 19 August 1352 without a male heir. The rule of the duchy was continued by his brother Wilhelm.