| Spinone Locum Ltd | Professional & scientific | 2005 |
| Spot Electrical Ltd | Construction | 2021 |
| Springhealth Ltd | Arts & recreation | 1999 |
| Spur Investments Limited | Real estate | 1960 |
| SRS Risk Consulting Ltd | Professional & scientific | 2011 |
| SRV Investments Limited | Real estate | 2019 |
| ST Wilfrids Road Limited | Households | 1992 |
| ST. Albans Enterprise Agency Limited | Professional & scientific | 1984 |
| ST. Albans Sub-Aqua Club Limited | Other services | 1981 |
| ST. Andrew's Court (Ware) Limited | Households | 1996 |
| ST. Christopher School (Letchworth) Limited | Education | 1954 |
| ST. Monica's Management Company (North) Limited | Households | 1974 |
| St.Albans Welding Co. Limited | | 1960 |
| Stackgrove Limited | | 1979 |
| Stansted Coaches Limited | | 1987 |
| Stapleton's (Tyre Services) Limited | Wholesale & retail | 1937 |
| Star Properties (UK) Limited | Real estate | 2002 |
| Starlight Plumbing Limited | Construction | 2003 |
| Steering Developments Limited | Manufacturing | 1991 |
| Stephen Buckle & Company LLP | | 2006 |
| Stephen Paul Byrne Limited | Construction | 1991 |
| Storm Coffee Co Limited | Wholesale & retail | 2022 |
| Storm Comms Ltd | Information & communication | 2015 |
| Storm Invest Limited | Wholesale & retail | 2020 |
| Storm-I.T. Limited | Information & communication | 2003 |
| Strongs & Bennett Limited | Professional & scientific | 1933 |
| Success Through People Ltd | Administrative & support | 2008 |
| SUE Miller Limited | Wholesale & retail | 2008 |
| Sungold Consultancy Limited | Information & communication | 1997 |
| Superverve Solutions Limited | Manufacturing | 2007 |
| Suretro Limited | | 1979 |
| Susie Hackett Communications Consulting Ltd | Professional & scientific | 2019 |
| Swordfish Property Investments Ltd | Extraterritorial | 2016 |
| Sylvan Heights Flat Management Limited | Real estate | 2002 |
| System Builders Limited | | 1989 |
| Systematic Services Limited | Construction | 2005 |
| T & T Publications Ltd | Information & communication | 2025 |
| T C Darby Machine Engraving and Screen Printing Company Limited | Other services | 2005 |
| TAN and Glam Limited | Other services | 2007 |
| Tantallon Management (Properties) Limited | Administrative & support | 2021 |
| Targasand (Contracts) Limited | Wholesale & retail | 1991 |
| TB Direct (Holdings) Limited | Financial services | 2006 |
| TB Surveying Services Ltd | Professional & scientific | 2019 |
| TBK Tiles Factory Outlet Limited | Wholesale & retail | 2017 |
| TBK Tiles Limited | Wholesale & retail | 2017 |
| TD Estates Services Ltd | Real estate | 2010 |
| Tech 2 Logistics Ltd | Transport & logistics | 2025 |
| Tecmark Limited | Information & communication | 2008 |
| Tegral Lighting Limited | Wholesale & retail | 2011 |
| Tetra Brickwork Solutions Ltd | Construction | 2019 |
| Thandy's Footsteps Ltd | Wholesale & retail | 2025 |
| THE Black Ostrich Company Ltd | Administrative & support | 2007 |
| THE Bloom Agency Ltd | Administrative & support | 2016 |
| THE Bowen Partnership LLP | | 2023 |
| THE Brondesbury RTM Company Ltd | Households | 2022 |
| THE Carpet Trading Company Limited | Transport & logistics | 1990 |
| THE Charles Kalms, Henry Ronson Immanuel College | Education | 1990 |
| THE Comic Alchemist Limited | Wholesale & retail | 2015 |
| THE DEW Drop Inn | | 1986 |
| THE Good Service Company Limited | Administrative & support | 2005 |
| THE Hale Club Limited | Arts & recreation | 1990 |
| THE John Richard Woodstock Corporation Limited | Extraterritorial | 2007 |
| THE Lilly ANN Care Services Ltd | Health & social care | 2020 |
| THE Market Theatre Company Limited | Arts & recreation | 2005 |
| THE Palace Theatre Watford Limited | Arts & recreation | 1996 |
| THE Partnerships Consultancy Ltd | Professional & scientific | 2023 |
| THE Priory Barn Events Ltd | Hospitality | 2025 |
| THE Query Tree (UK) Ltd | Information & communication | 2012 |
| THE Riverside Garden Centre Limited | Wholesale & retail | 1997 |
| THE School of Icasa Limited | Education | 2006 |
| THE Storage Centre Limited | Construction | 2003 |
| THE Tutoring Standards Company Limited | Education | 2015 |
| THE Vasiliou Collective EOT Management Limited | Real estate | 2023 |
| THE Very Small Building Company Limited | Construction | 2007 |
| THEORY7 Limited | Information & communication | 2001 |
| Third Scale Technology Limited | | 1978 |
| Thomas Heskin Court Limited | Households | 1985 |
| Thornmate Limited | | 1987 |
| Thornton Logistics Limited | Transport & logistics | 2007 |
| Those That CAN Limited | Other services | 2019 |
| Threads of Harpenden Limited | Wholesale & retail | 2004 |
| Thriplow Farms Limited | Agriculture & fishing | 1939 |
| Tile Tech Walls and Floors Limited | Wholesale & retail | 2006 |
| Tilehouse Properties Limited | Real estate | 1952 |
| Tiles & Baths Direct Limited | Construction | 2001 |
| TIM Brading Architect Ltd | Professional & scientific | 2005 |
| Titleline Limited | | 1985 |
| TLC Pet SIT Ltd | Other services | 2017 |
| TLC Pet Sitting Ltd | Other services | 2017 |
| TM Medical Consultants Limited | Health & social care | 2004 |
| TMS 2012 Limited | Administrative & support | 2012 |
| TOM Cummins Limited | Transport & logistics | 2001 |
| Tomlin Builders Limited | Construction | 1985 |
| Tony C K Wong Limited | Professional & scientific | 2006 |
| Topsharp Limited | Wholesale & retail | 1991 |
| Topsy Turvy Designs Limited | Real estate | 1992 |
| Totem Care Limited | Financial services | 2006 |
| Tour Movements Chauffeurs Ltd | Transport & logistics | 2026 |
| Tower Kitchens Limited | Construction | 2016 |
| Tracy Cook Consulting Ltd | Information & communication | 2014 |