TSI
- Remedial education
- TSI slant
- Technical Specifications for Interoperability
- Technology Schools Initiative
- TekSavvy
- Telesensory Systems
- Televisione svizzera di lingua italiana
- The Seasteading Institute
- Three Seas Initiative
- Thyroid stimulating immunoglobulin
- Time in Turkey
- Time-Slot Interchange
- Torres Strait Islanders
- Torres Strait Islands
- Total solar irradiance
- Total system inertia
- Town Sports International
- Trading Standards Institute
- Trauma symptom inventory
- Trophic state index
- True strength index
- Trustworthy Software Initiative
- Turbo fuel stratified injection
- Turkish Standards Institution