Nieder-Isenburg (often called Lower Isenburg) was a small mediaeval county in northern Rhineland-Palatinate, Germany.

It was located to the east of the town of Neuwied, due north of Vallendar.

Following the death of Count Ernst of Isenburg in Brussels in 1664 without direct heir, the territories of Nieder-Isenburg were claimed back as a feudal tenure by the Archbishopric of Cologne, the Archbishopric of Trier and the core tenure of Isenburg and Grenzau by the Archbishopric of Fulda.

Eventually the Walderdorff estates were distributed to the Nassovian state in the Napoleonic era.

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