NSI may refer to:

• National Student Index, a system that assigns every New Zealand secondary and tertiary student with a unique identification number.
• 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.
• Nutraceutical Sciences Institute, a company which produces and markets its own line of vitamins and nutrition supplements.
• New Slovenia Christian People's Party, a right-of-centre political party in Slovenia.
• National systems of innovation
• National Savings and Investments, a state-owned savings bank in the United Kingdom.
• Northern Sydney Institute of TAFE
• the IATA airport code of Yaoundé Nsimalen International Airport
• NSÍ Runavík, a Faroese football (soccer) team

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