YSI may refer to:
- Young Scots for Independence
- YouSendIt, an online site providing sending of large files
- Young Social Innovators, a project-based competition for schools in the Republic of Ireland
- Youth Science Ireland, formerly IAYSG (Irish Association of Youth Science Groups)
- YSI Inc., a brand of sensors and software for Xylem, Inc.
- YSi, a silicide composed of yttrium and silicon
- Parry Sound/Frying Pan Island-Sans Souci Water Aerodrome (IATA code: YSI), a Canadian airport