| Tiligo Oy | Professional & scientific | 2006 |
| Tilintarkastustoimisto Conaudit Oy | Professional & scientific | 1996 |
| Tilitoimisto Konttokurantti Oy | Professional & scientific | 1998 |
| Tilitoimisto Scandea Oy | Professional & scientific | 2013 |
| Timanttipyörä Oy | Other services | 2025 |
| Timedi Oy | Wholesale & retail | 1998 |
| Timeless design Oy | Wholesale & retail | 2021 |
| Tin Sheung Oy | Hospitality | 1995 |
| Tirax Oy | Other services | 2002 |
| Tivoliburger Ky | Hospitality | 2011 |
| Toiminnantarkastus SaraLeena avoin yhtiö | Professional & scientific | 2022 |
| Tolt Traders Oy | Wholesale & retail | 2017 |
| Tommin Apu Oy | Wholesale & retail | 2020 |
| Tonefest Enterprises Oy | Arts & recreation | 2023 |
| Tonoman Medical Instruments TMI Ltd Oy | Professional & scientific | 2008 |
| TonyFix Oy | Other services | 2026 |
| Toomiles Entertainment Oy | Information & communication | 2020 |
| Top Manual Oy | Professional & scientific | 2010 |
| Topicross Ky | Arts & recreation | 2014 |
| Toppap Oy | Administrative & support | 2020 |
| Tor-Trading Oy | Wholesale & retail | 1992 |
| Total Effect Oy | Professional & scientific | 2022 |
| Totalrent Oy | Administrative & support | 2023 |
| Totum Oy | Professional & scientific | 2010 |
| Training Club Tuusula Oy | Arts & recreation | 2012 |
| Trano Oy | Professional & scientific | 2015 |
| Trantech Oy | Wholesale & retail | 1998 |
| Travel Design Store Collection Oy | Professional & scientific | 2019 |
| Travel S Oy | Administrative & support | 2015 |
| Tree Amigos Oy | Wholesale & retail | 2019 |
| Tree of Pets Oy | Wholesale & retail | 1990 |
| Trevor Group OY | Transport & logistics | 2022 |
| TriaMed Oy | Education | 2025 |
| Triisilmä Oy | Health & social care | 2018 |
| Triwest Oy | Construction | 1998 |
| Trä Group Oy | Administrative & support | 2017 |
| Trälådor Oy Ab | Construction | 2018 |
| Tuija Ukonmäki Oy LKV Isännöintipalvelut | Real estate | 2011 |
| Tuive Oy | Financial services | 2018 |
| Tujaus Oy | Information & communication | 2007 |
| TulosVelho Oy | Professional & scientific | 2026 |
| Tumik Oy | Professional & scientific | 2008 |
| Tuontiliike Rimini Oy | Real estate | 2000 |
| Tuotannon Varmistus Oy | Manufacturing | 1976 |
| Tuplatilit Oy | Professional & scientific | 2023 |
| Turhake Oy | Manufacturing | 2020 |
| TurvaPro K&K avoin yhtiö | Administrative & support | 2016 |
| Tuulentien Kulkijat Oy | Wholesale & retail | 2022 |
| Tuulianna Invest Oy | Financial services | 2016 |
| Tuulikallio Group Oy | Financial services | 2025 |
| Tuuma Digital Oy | Professional & scientific | 2025 |
| Tuus - Catering Oy | Hospitality | 2014 |
| Tuus Kokkaa & Leipoo Oy | Wholesale & retail | 2024 |
| Tuus-Tec Ky Leinonen | Construction | 2006 |
| TuusGroup Oy | Construction | 2023 |
| TuusKlinikka Oy | Health & social care | 1992 |
| Tuusdata Oy | Transport & logistics | 1997 |
| Tuusfix Oy | Construction | 2022 |
| Tuushoiva Oy | Administrative & support | 2023 |
| Tuusmet Oy | Manufacturing | 2006 |
| Tuusmotor Oy | Wholesale & retail | 2005 |
| Tuuspoint Oy | Other services | 2013 |
| Tuusport Oy | Wholesale & retail | 1993 |
| Tuusrak Oy | Other services | 2025 |
| Tuusreal Oy | Real estate | 2012 |
| Tuusrent Oy | Construction | 2022 |
| Tuusulan ASR-Palvelu OY | Health & social care | 2002 |
| Tuusulan Auto-Centre Oy | Other services | 1987 |
| Tuusulan Autoexpertti Oy | Wholesale & retail | 2022 |
| Tuusulan Autohuolto Oy | Other services | 2008 |
| Tuusulan Autokorjaamo Oy | Other services | 2021 |
| Tuusulan Autopesu Oy | Other services | 2021 |
| Tuusulan Autopojat Oy | Other services | 2023 |
| Tuusulan Autorakenne Oy | Other services | 1984 |
| Tuusulan Car Korjaamo Oy | Other services | 2018 |
| Tuusulan EKL Palvelut Oy | Transport & logistics | 1990 |
| Tuusulan Energia Oy | Energy | 1986 |
| Tuusulan Golfklubi Oy | Arts & recreation | 1994 |
| Tuusulan Hammaslääkäriasema Oy | Health & social care | 2007 |
| Tuusulan Hihna Oy | Manufacturing | 2025 |
| Tuusulan Högmalm ky | | 1984 |
| Tuusulan Höyrypesupalvelu Oy | Other services | 2025 |
| Tuusulan Jäähalli Oy | Arts & recreation | 1996 |
| Tuusulan Kaivuu ja Kuormaus Oy | Agriculture & fishing | 2003 |
| Tuusulan Kalamiehenpuisto Oy | Real estate | 1996 |
| Tuusulan Keilahalli Oy | Arts & recreation | 2009 |
| Tuusulan Kylmähuolto Oy | Construction | 2025 |
| Tuusulan LVI Palvelu Oy | Construction | 2011 |
| Tuusulan Laaturakenne Oy | Construction | 1989 |
| Tuusulan Lukko Oy | Wholesale & retail | 2006 |
| Tuusulan MR-Sähkö Oy | Construction | 2007 |
| Tuusulan Maansiirto Oy | Construction | 1986 |
| Tuusulan Maaperätutkimus Oy | Professional & scientific | 1999 |
| Tuusulan Metalli ja Koneistuspalvelut Oy | Manufacturing | 2023 |
| Tuusulan Mopoautot Oy | Wholesale & retail | 2010 |
| Tuusulan Optiikka Oy | Wholesale & retail | 2013 |
| Tuusulan Parturi Mars Oy | Other services | 2025 |
| Tuusulan Pelti ja Lumi Oy | Construction | 2006 |
| Tuusulan Peltikeskus Oy | Construction | 1994 |
| Tuusulan Remonttipalvelu Oy | Construction | 2011 |