Gerlach III of Isenburg-Grenzau was the Count of Isenburg-Grenzau from 1502 until 1530. Gerlach was a son of Count Gerlach II of Lower Isenburg. In 1495 he was chosen to hold the Imperial banner at the Reichstag in Worms, greatly improving the renown of his family. In 1502 his father died, partitioning his territories between Gerlach and his brother Salentin VI (whose state was named Isenburg-Neumagen). Gerlach died in 1530 and was succeeded by his son Henry.