Nienburg, meaning "new castle", may refer to the following locations in Germany:
- Nienburg (district), Lower Saxony
- Nienburg, Lower Saxony
- Nienburg station
- Nienburg II – Schaumburg, a German parliament constituency
- Nienburg, Saxony-Anhalt
- Nienburg Abbey, a Benedictine monastery
- Nienburg (Verwaltungsgemeinschaft), a collective municipality, disbanded in 2010