Nj or NJ may stand for:
- New Jersey, the state in the United States, nicknamed "Garden State"
- Nanojoule (nJ), an International System of Units (SI) unit of energy equal to 10−9 joules
- Nj, a diagraph
- Narva-Jõesuu, Estonia
- Nordjyske Jernbaner, a Danish railway
- Neighbor-joining, a bioinformatic method for the construction of phylogenetic trees
- Napierville Junction Railway