Eric I, Duke of Brunswick-Grubenhagen nicknamed the winner (German: Erich I., Herzog von Braunschweig-Grubenhagen; c. 1383 – 28 May 1427), ruled the Principality of Grubenhagen, a part of the Duchy of Brunswick-Lüneburg.
Disputes arose with his cousins Bernard I and Frederick I of Lüneburg, with the city of Brunswick, with his former guardian, with the counts of Schwarzburg and the Landgrave of Thuringia.
In 1415, Eric declared war on the Counts of Hohenstein, because both sides claimed the county of Bad Lauterberg im Harz and defeated them near the village of Osterhagen.
In 1422, the Abbess of Gandersheim Abbey (who happened to be his daughter, Agnes II of Brunswick-Grubenhagen) enfeoffed him with castle and city of Elbingerode.
Two years, in 1424, the Abbess of Quedlinburg Abbey (Countess Adelheid of Isenburg), enfeoffed the Dukes of Grubenhagen with Duderstadt, Gieboldehausen and the Golden Mark.