IIN and variants may mean:
- II-n or IIn, a subtype of Type II supernova
- IIN, the Issuer Identification Number, see ISO/IEC 7812
- IIN, the ICAO airline code for Inter Islands Airlines
- IIN, the IATA airport code for Nishinoomote Airport or Tanagashima Airport, see Ōsumi Islands
- IIN, a model of airplane manufactured by Nieuport
- iin, the stock ticker symbol on the Australian Securities Exchange for iiNet