NII may refer to one of the following.
- National Information Infrastructure, a telecommunications policy buzzword, coined under the Clinton Administration in the United States
- Necessary International Initiative
- Nuclear Installations Inspectorate, part of the UK Office for Nuclear Regulation
- NII Holdings, Inc. (f/k/a Nextel International, Inc.), a wireless communications company with operations in Latin America
- NII, the transliteration of the Russian language abbreviation for "scientific research institute" (nauchno-issledovatelskiy institut, научно-исследовательский институт)
- National Institute of Informatics, a Japanese research institute for the advancement of studies in informatics
- (In finance) Net Interest Income
- National Institute of Immunology
- Non-Intrusive Inspection of cargo containers
- Nii means "king" in the Ga language of Ghana