YSI may refer to: 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 YSI Includes created by Y_Less for SA-MP, a multiplayer modification of GTA Sanandreas.