| Signified Oy | Administrative & support | 2021 |
| Siito-Vuori Oy | Mining & quarrying | 1988 |
| Sijoitus-B Oy | Financial services | 2008 |
| Siveb Innovations & Application Oy Ab | Information & communication | 2018 |
| Siveb oy ab | Professional & scientific | 2015 |
| Skagsund Ab | Agriculture & fishing | 1989 |
| SkarpTech Oy | Information & communication | 2025 |
| Skräbböle bryggeri ab | Manufacturing | 2021 |
| Skärgårdens Bygghjälp Ab - Saariston Rakennusapu Oy | Construction | 2020 |
| Skärgårdens El-Lås Kb | Construction | 1991 |
| Skärgårdens Gräv & Transport Ab - Saariston Kaivuu & Kuljetus Oy | Construction | 2012 |
| Skärgårdens Modulhus Ab Oy | Construction | 2019 |
| Skärgårdsstallet Ab | Education | 2018 |
| Skådeborg Kb | Hospitality | 1982 |
| Smart-Paint Oy | Construction | 2025 |
| Snåre consulting Oy Ab | Financial services | 2026 |
| Solertiax Oy | Wholesale & retail | 2012 |
| Solliden Camping OY AB | Administrative & support | 1993 |
| Solobroker Oy | Information & communication | 2026 |
| Solutions for water pollution Ab | Professional & scientific | 2023 |
| Sprinkly Rice Ab | Hospitality | 2019 |
| Stormälön Mariina Oy | Transport & logistics | 1989 |
| Storskär Oy | Financial services | 2026 |
| Sunis Fixing Ab Oy | Construction | 2023 |
| Suomen Kynttilälähetit Oy | Wholesale & retail | 2022 |
| Suomen Venevakuutus Oy | Financial services | 2025 |
| Suomisen Rengas kommandiittiyhtiö | Other services | 1986 |
| Suunnittelutoimisto Taika Ky | Professional & scientific | 2012 |
| Suvi Satama Oy | Transport & logistics | 2024 |
| Sydwest Properties Oy Ab | Real estate | 2021 |
| Sähkö T. Lehto Oy | Construction | 2016 |
| Sähkö Valox Oy | Construction | 2018 |
| Sähköputsaus Ky | Administrative & support | 2004 |
| Sähkötekniikka Heinonen Oy | Construction | 2025 |
| T & S Hansén Ab | Administrative & support | 2011 |
| T. Kallinen Oy | Administrative & support | 2019 |
| TR-Kone Oy | Administrative & support | 1989 |
| TSM Rakennustelineet Oy | Administrative & support | 2025 |
| Taimiemo Oy | Agriculture & fishing | 2018 |
| Taimilla Oy | Construction | 2024 |
| Tallbacka No Dig Oy Ab | Agriculture & fishing | 2024 |
| Tallitila Oy | Real estate | 2022 |
| Talorum Oy | Real estate | 1991 |
| Talousverkko Yritystilit Oy | Professional & scientific | 2009 |
| Tant Charlotta Ab Oy | Health & social care | 2022 |
| Tapitus Oy | Construction | 2018 |
| Taxi Elovirna Ab Oy | Transport & logistics | 2005 |
| Tebescon Ab Oy | Construction | 1991 |
| Tefire Oy | Construction | 1986 |
| Teija Laaksonen Oy | Professional & scientific | 2026 |
| Teknovisio Oy | Professional & scientific | 2016 |
| Temppelin Emäntä Oy | Education | 2021 |
| Tepukax Oy | Wholesale & retail | 2020 |
| The Unusual Company Oy | Professional & scientific | 2019 |
| Therapy EP Oy | Health & social care | 2023 |
| Three Times Forty Oy | Hospitality | 2022 |
| Tiivakka Oy | Education | 1996 |
| Tikula Holding Oy | Financial services | 2019 |
| Tili Sydwest Oy Ab | Professional & scientific | 1989 |
| Transportfirma Donner Ab | Transport & logistics | 1972 |
| Transportfirma T & G Söderholm Ab | Transport & logistics | 2004 |
| Transportfirma Wihlman Kb | Transport & logistics | 1982 |
| Tri Amigos Los Glassos Oy | Wholesale & retail | 2022 |
| Triocon Oy | Wholesale & retail | 2007 |
| Triple8 Oy | Financial services | 2005 |
| True Contact Oy | Other services | 2022 |
| Tuli Group Oy | Information & communication | 2012 |
| Tulitalo avoin yhtiö | Construction | 1996 |
| Tulkkaus Sointuvi Oy | Professional & scientific | 2024 |
| TunturiTec Electric Ab | Construction | 1994 |
| TunturiTec Oy | Manufacturing | 2004 |
| Tuonti ja tukku Special Oy | Manufacturing | 1993 |
| Tuotantoyhtiö Kalkki Oy | Arts & recreation | 2024 |
| Turi Invest Oy | Transport & logistics | 2019 |
| Turi Rakennus Oy | Construction | 2020 |
| Turun Pelti- ja Metallityö Oy | Construction | 1991 |
| Turun Saaristorakentajat Oy | Professional & scientific | 2018 |
| Turun asuntoauto vuokraus Oy | Administrative & support | 2019 |
| Turunmaan Kiinteistöpalvelut Oy | Administrative & support | 2025 |
| Ulkoiluaitta Oy | Wholesale & retail | 1952 |
| Urakointi Wester Ky | Construction | 2021 |
| V-A Kiinteistöhallinta Oy | Real estate | 2026 |
| V-A Talotekniikka Oy | Construction | 2019 |
| V. Viloma Oy Ab | Professional & scientific | 2025 |
| VMK Solutions Oy | Professional & scientific | 2023 |
| VOIDSTAR Ab | Information & communication | 2022 |
| VRA Physiotherapy and Health Ab | Health & social care | 2025 |
| VS Rakennustiimi Oy | Construction | 2024 |
| Vaapukka Group Oy | Financial services | 2025 |
| Valoniemen laituriosuuskunta | Transport & logistics | 2014 |
| Valoniemi Ventures Oy | Financial services | 2025 |
| Valorum Oy | Real estate | 2019 |
| Varrak Oy | Construction | 1990 |
| Varsinais-Suomen Saaristonrakentaja Oy | Construction | 2022 |
| Venetukku Oy | Wholesale & retail | 1962 |
| Verkkokangas Oy A&M | Wholesale & retail | 2009 |
| Verkstad Risto Eriksson Oy Ab | Professional & scientific | 1997 |
| Victoria Invest Oy Ab | Wholesale & retail | 2018 |
| Viipus Investment Oy | Financial services | 2018 |
| Villa Reuter Oy Ab | Real estate | 1990 |