| THE RAD Brothers Limited | Administrative & support | 2020 |
| THE Really Simple Group Ltd | Information & communication | 2013 |
| THE Recover Team Limited | Health & social care | 2013 |
| THE Red Rubber Ball Foundation | Other services | 2010 |
| THE Rooms GG Limited | Real estate | 2025 |
| THE Salon Welwyn Limited | Other services | 2018 |
| THE Shred Shed Ltd | Education | 2025 |
| THE Tattooed Barber Ltd | Other services | 2019 |
| THE Teesport Limited Partnership | | 2008 |
| THE Tesco Aqua Limited Partnership | | 2006 |
| THE Tesco Atrato Limited Partnership | | 2009 |
| THE Tesco Blue Limited Partnership | | 2006 |
| THE Tesco Coral Limited Partnership | | 2008 |
| THE Tesco Dorney Limited Partnership | | 2012 |
| THE Tesco Kirkby Limited Partnership | | 2007 |
| THE Tesco Navona Limited Partnership | | 2010 |
| THE Tesco Passaic Limited Partnership | | 2010 |
| THE Tesco Property Limited Partnership | | 2003 |
| THE Tesco Red Limited Partnership | | 2006 |
| THE Tesco Sarum Limited Partnership | | 2011 |
| THE Vehicle Parts GUY UK Ltd | Wholesale & retail | 2023 |
| THE Wellness Box Ltd | Arts & recreation | 2020 |
| THE Welwyn Group Limited | Administrative & support | 2008 |
| THE Whitwell HOO Partnership Limited | Construction | 1991 |
| Thebez Global Resources Limited | Administrative & support | 2020 |
| Thejkl Limited | Health & social care | 2024 |
| Therapeutic Hooves CIC | Health & social care | 2019 |
| Thirteen Consulting Ltd | Professional & scientific | 2022 |
| Three Clover Care Limited | Administrative & support | 2020 |
| Three Magnets Arts Ltd | Education | 2009 |
| Three Magnets Centre Limited | Other services | 2015 |
| Three Magnets Limited | Extraterritorial | 2009 |
| Tightline Audio Limited | Manufacturing | 2014 |
| Timber to Energy Ltd | Water & waste | 2025 |
| Timbre & Calibre Ltd | Manufacturing | 2021 |
| Timeless Gardens Ltd | Administrative & support | 2018 |
| Timeless Services Ltd | Professional & scientific | 2023 |
| Timocicu Limited | Construction | 2015 |
| Tiny Giants Pre-School Ltd | Education | 2019 |
| Tiny Herts Babywear Ltd | Wholesale & retail | 2013 |
| Tinytumsrecipes Limited | Other services | 2021 |
| Titus Beauchamp Limited | Financial services | 2026 |
| TJI Investment Properties Ltd | Real estate | 2023 |
| Tjlrusselllocs Ltd | Information & communication | 2022 |
| TK Home Improvements Limited | Construction | 2024 |
| TMEM16A Limited | Other services | 2020 |
| TMG Electrics Ltd | Construction | 2021 |
| TML Consultancy Ltd | Information & communication | 2014 |
| Tobys Pest Control Ltd | Administrative & support | 2025 |
| Toffee Hair& Beauty House Limited | Other services | 2022 |
| Together for Welwyn Garden City | Arts & recreation | 2006 |
| TOM Harry Ltd | Construction | 2017 |
| Toolkitchenhub Ltd | Wholesale & retail | 2024 |
| Top Flight Solutions Ltd | Administrative & support | 2023 |
| Total Care and Compliance Services Ltd | Health & social care | 2014 |
| Totalfix Maintenance Ltd | Construction | 2025 |
| Toti Management Limited | Hospitality | 2024 |
| Touchstone Rail Ltd | Professional & scientific | 2022 |
| Tower Cranes UK Limited | Construction | 2006 |
| TPI Fund Managers Limited | Financial services | 2018 |
| TPT Holdco NO.1 Limited | Real estate | 2008 |
| Tracey Stowe Travel Limited | Administrative & support | 2015 |
| Track Safe Limited | Construction | 2022 |
| Tradebrain Systems Ltd | Information & communication | 2025 |
| Tramping Ltd | Construction | 2018 |
| Transcend Retail Solutions Limited | Manufacturing | 2023 |
| Transfans Ltd | Professional & scientific | 2025 |
| Transport Planning Associates Limited | Professional & scientific | 1997 |
| Trauma Recovery CIC | Health & social care | 2020 |
| Travel More Ltd | Transport & logistics | 2020 |
| Travelyields Limited | Professional & scientific | 2011 |
| Treci Ltd | Information & communication | 2025 |
| Trellis Consulting Limited | Professional & scientific | 2026 |
| Tribe Advisory Limited | Other services | 2017 |
| Tribe Risktrack Limited | Information & communication | 2025 |
| Tribe Schemes Limited | Professional & scientific | 2017 |
| Trinity Design Studio Limited | Wholesale & retail | 2020 |
| Tristar Tech Solutions Limited | Information & communication | 2000 |
| Tromedia Limited | Professional & scientific | 2023 |
| Trundletrading Ltd | Extraterritorial | 2025 |
| Truqa Limited | Information & communication | 2021 |
| Tsfm Limited | Manufacturing | 2022 |
| TTO Properties Ltd | Real estate | 2024 |
| Tufko Ltd | Information & communication | 2025 |
| Turcuisine Welwyn Limited | Real estate | 2020 |
| Turner and Betts Properties Ltd | Households | 2026 |
| Turquoise Noise Ltd | Professional & scientific | 2005 |
| Tusk Therapeutics Ltd | Professional & scientific | 2015 |
| TWENTYSE7EN Ltd | Wholesale & retail | 2011 |
| Twinroots Landscapes Ltd | Administrative & support | 2025 |
| Two Carrots Studio Ltd | Information & communication | 2024 |
| Two Lads Productions Ltd | Information & communication | 2025 |
| TWS Building Services Ltd | Water & waste | 2024 |
| TXW Limited | Financial services | 2016 |
| Tyler Fencing Limited | Wholesale & retail | 2020 |
| Tylers AC Contractors Limited | Construction | 2025 |
| U&U Development Limited | Real estate | 2026 |
| Ucar Electric & Building Ltd | Professional & scientific | 2022 |
| Ucomply Limited | Information & communication | 2011 |
| Udybless Limited | Health & social care | 2017 |