Ernest I of Brunswick-Grubenhagen

Ernest I of Brunswick-Grubenhagen (c. 1297 – 9 March 1361) was a younger son of Duke Henry I the Admirable, who founded the principality of Grubenhagen in 1291 and left eight sons and eight daughters. Destined for the Church, Ernest after his father's death in 1322 ruled jointly with his brothers Henry II and William.

A pact of 1324 soon gave way to partition: Ernest received the area around Einbeck, Grubenhagen Castle and Osterode, while several towns stayed under joint rule. Henry II withdrew in 1342 and died in 1351, and when William died childless in 1360 Ernest ruled the whole principality.

He had made his son Albert I co-regent in 1359, and when he died on 9 March 1361 Albert succeeded him.