John of Isenburg-Grenzau (German: Johann V. von Isenburg-Grenzau) (c. 1507 – 18 February 1556, in Montabaur) was the Count of Isenburg-Grenzau from 1554 until 1556, and (as John IV) the Archbishop-Elector of Trier from 1547 until 1556. John spent much of his time in his Westerwald residence of Montabaur, where he died in 1556.