| Image Coa: | Wappen Niedersayn.svg |
| Coordinates: | 50.5303°N 7.8319°W |
| Image Plan: | Niedersayn im Westerwaldkreis.png |
| State: | Rheinland-Pfalz |
| District: | Westerwaldkreis |
| Verbandsgemeinde: | Wirges |
| Elevation: | 340 |
| Area: | 2.82 |
| Postal Code: | 56244 |
| Area Code: | 02602 |
| Licence: | WW |
| Gemeindeschlüssel: | 07 1 43 275 |
| Divisions: | 3 |
| Website: | www.niedersayn.de |
| Mayor: | Günter Anton Kober[1] |
| Leader Term: | 2019 - 24 |
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 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.
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.