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