NSI may refer to:
- NASA Standard Initiator, an electrochemical device which plays a critical role in initiating various pyrotechnic events in the National Space Transportation System.
- National Savings and Investments, a state-owned savings bank in the United Kingdom.
- National Screen Institute, a non-profit organization headquartered in Winnipeg, Manitoba, Canada
- National Security Institute
- National Security Intelligence
- National Space Institute
- National Student Index, a system that assigns every New Zealand secondary and tertiary student with a unique identification number.
- Nationwide Suspicious Activity Reporting Initiative
- Network Service Interface, the interface between a station and the network in a mobile communications network.
- Network Solutions, LLC., a technology company founded in 1979.
- Neurosciences Institute, a nonprofit research institute that focuses on research designed to discover the biological basis of higher-brain function.
- New Slovenia – Christian People's Party, a right-of-centre political party in Slovenia.
- National systems of innovation
- Northern Sydney Institute of TAFE
- the IATA airport code of Yaoundé Nsimalen International Airport
- NSÍ Runavík, a Faroese football (soccer) team