Niedersayn is an Ortsgemeinde – a community belonging to a Verbandsgemeinde – in the Westerwaldkreis in Rhineland-Palatinate, Germany.

The community lies north of Montabaur on the river Sayn.

Since 1971 it has belonged to what was then the newly founded Verbandsgemeinde of Wirges, a kind of collective municipality.

The municipal council is made up of 7 council members, including the extraofficial mayor (Bürgermeister), who were elected in a majority vote in a municipal election on 13 June 2004.

The nearest InterCityExpress stop is the railway station at Montabaur on the Cologne-Frankfurt high-speed rail line.