| Teilo Developments Limited | Construction | 1985 |
| Teilo Holdings Ltd. | Professional & scientific | 2025 |
| Teilo Scaffolding Ltd | Construction | 2016 |
| Teilo Vets Ltd | Professional & scientific | 2010 |
| Terry Davies Plumbing & Heating Limited | Construction | 2013 |
| THE Catering and Cocktail Co Ltd | Hospitality | 2022 |
| THE Chippery Llandeilo Ltd | Hospitality | 2023 |
| THE Class House Llandeilo Limited | Arts & recreation | 2020 |
| THE Country Retreat Llandeilo Ltd | Real estate | 2018 |
| THE Dahlia Wood Ltd | Wholesale & retail | 2022 |
| THE Garden Group Limited | Hospitality | 2022 |
| THE Happy Herd Company Ltd | Agriculture & fishing | 2020 |
| THE Little Welsh Dresser Ltd | Extraterritorial | 2014 |
| THE Pink Peppercorn Ltd. | Manufacturing | 2018 |
| THE Plough at Rhosmaen Limited | Hospitality | 2004 |
| THE Rook Cafe Ltd | Wholesale & retail | 2026 |
| THE Trust for Welsh Archaeology | Professional & scientific | 1975 |
| THE Wessex Brewery Limited | Manufacturing | 2000 |
| Thomas's Building Contractors Ltd | Construction | 2011 |
| Tirylan House Retreat Centre | Hospitality | 2022 |
| TLB Planning Ltd | Professional & scientific | 2019 |
| Top HAT Finance Ltd | Professional & scientific | 2023 |
| Tower Wealth Management Ltd | Financial services | 2023 |
| Towy Metals Ltd | Water & waste | 2017 |
| Treadwise Limited | Wholesale & retail | 2025 |
| Treehouse Community Sustainability and Wellbeing C.I.C. | Education | 2024 |
| Treharne Homes Ltd. | Construction | 2008 |
| Trywydd Cyfyngedig | Administrative & support | 2009 |
| TS Marine Electrical Services Ltd | Manufacturing | 2019 |
| Two Oaks Estates Ltd | Real estate | 2025 |
| Twts Tywi Limited | Health & social care | 2020 |
| Tywi Group Ltd | Health & social care | 2025 |
| Tywi Tactical Ltd | Wholesale & retail | 2023 |
| Tywi Valley Paving Limited | Administrative & support | 2019 |
| Vanguard Transport Ltd | Transport & logistics | 2018 |
| Ventura SOL Limited | Real estate | 2013 |
| Virtual Ward Technologies Covid Ltd | Information & communication | 2020 |
| Virtual Ward Technologies Limited | Professional & scientific | 2020 |
| W & BA Carpets & Blinds Limited | Wholesale & retail | 2002 |
| Wayne Howells Plant Hire Limited | Administrative & support | 2017 |
| Wellinbyte Limited | Hospitality | 1998 |
| Wesaveumoney Limited | Extraterritorial | 2015 |
| White Hart Llandeilo Catering Ltd | Hospitality | 2025 |
| Wild About Gardens Limited | Administrative & support | 2013 |
| William Corbett & Co. Limited | Manufacturing | 1991 |
| WJ01 Promotions Limited | Arts & recreation | 2020 |
| Wotwe Ltd | Arts & recreation | 2022 |
| Yann River Limited | Information & communication | 1994 |
| Yarn and SEW Ltd | Wholesale & retail | 2023 |
| Ymddiriedolaeth Y Neuadd Ddinesig Llandeilo Fawr Civic Hall Trust Ltd. | Information & communication | 2005 |
| YR Hafod New Road Management Company Limited | Households | 2000 |