Philip I, Duke of Brunswick-Grubenhagen (German: Philipp I., Herzog von Braunschweig-Grubenhagen; 1476 – 4 September 1551, Herzberg) was a member of the House of Guelph.

He was the second son of Duke Albert II of Grubenhagen and his wife Elizabeth, née Countess of Waldeck.

He then reformed the monasteries in his principality and in 1538, he adopted a Church order for Grubenhagen and declared the papal doctrine to be abolished.

In 1546 Philip and his sons participated in a military campaign in the Schmalkaldic War in southern Germany, which ended unsuccessfully at Ingolstadt.

This campaign drew the ire of Emperor Charles V. After the utter defeat of the Protestants, he was acquitted in 1548 and reinstated as Duke.