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. Its seat is in the like-named town.
Niedersayn’s Ortsteile are Karnhöfen, Blaumhöfen and Niedersayn.
In 1277, Niedersayn had its first documentary mention under the name Langesain.
The 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.
East of the community runs Bundesstraße 255, linking Montabaur and Rennerod, and also Bundesstraße 8. The nearest Autobahn interchange is Montabaur on the A 3 (Cologne–Frankfurt), some 3 km away. The nearest InterCityExpress stop is the railway station at Montabaur on the Cologne-Frankfurt high-speed rail line.