Ni or NI may be:

• A region of the United Kingdom, see Northern Ireland.
• A part of the United Kingdom social security system, see National Insurance.
• A US-based measurement and automation company, see National Instruments.
• A top level domain and ISO country code for Nicaragua.
• the chemical abbreviation for Nickel (Ni).
• A catch phrase: see the Knights who say Ni
• Ni is the third syllable in Buddhist "Six syllable mantra" Om mani padme hum.
• The number 2 (二 ) in Japanese
• The Swedish word Ni
• Ni , a book by Saryan Kim
• One of the Secret Words. Ni, Peng and Nee-Wong