Philip I of Isenburg-Grenzau (German: Philipp I. von Isenburg-Grenzau) was the Count of Isenburg-Grenzau from 1341 until 1361. Philip came into conflict with the expansionist prince-Archbishop Baldwin of Luxembourg of Trier. In 1346 Baldwin expanded his power deep into the Westerwald, and the following year Philip allied with Count Reinhard I of Westerburg against him. It began the Grenzauer Feud which was only ended after the intervention of the Emperor. In 1361, Philip was forced to acknowledge the overlordship of the Archbishops shortly before his death. He was succeeded by Eberhard II in 1361.