| Specialist Aviation Limited | Transport & logistics | 1995 |
| Specialist Aviation Manufacturing Limited | Manufacturing | 1974 |
| Specialist Aviation Spares Limited | Professional & scientific | 2009 |
| Specialized Security Products Limited | Wholesale & retail | 2000 |
| Specifyai Ltd | Information & communication | 2025 |
| Spheron Architects Limited | Professional & scientific | 2011 |
| Spicefair Ltd | Wholesale & retail | 2012 |
| Sportandhealth Ltd | Education | 2019 |
| Spruse Cleaning Services Ltd | Administrative & support | 2024 |
| Square Tomato Limited | Manufacturing | 1999 |
| Squire John Holdings Limited | Real estate | 1989 |
| Srana Estates Limited | Real estate | 2026 |
| SRI Guru Singh Sabha Gurdwara Letchworth | Other services | 2005 |
| SRI Guru Singh Sabha Hall Limited | Extraterritorial | 2005 |
| Ssjt Ltd | Professional & scientific | 2019 |
| ST Georges House Limited | Real estate | 2023 |
| ST. Francis Court Management (NO.1) Limited | Households | 1987 |
| ST. Michaels Road Hitchin Residents Association Limited | Households | 1996 |
| St.Mary's Close Residents'Association Limited | Households | 1969 |
| Stanbor Limited | Real estate | 2019 |
| Starform Printing Ltd | Manufacturing | 2010 |
| Starlite Transport Limited | Transport & logistics | 2025 |
| Steeldeck Enterprise Limited | Manufacturing | 2010 |
| Steeldeck Hall Stage Ltd | Manufacturing | 2018 |
| Steeldeck Industries Ltd | Manufacturing | 1995 |
| Stellaone Ltd | Education | 2012 |
| Stephen Hensby Associates Limited | Real estate | 2003 |
| Stepping Stones Letchworth Ltd | Wholesale & retail | 2023 |
| Sterlingside Capital Holdings Limited | Financial services | 2023 |
| Stewart Croft Freeholders Ltd | Households | 2001 |
| Stewarts Croft Residents Association Limited | Other services | 1988 |
| Stillwater Bathrooms Ltd | Wholesale & retail | 2025 |
| Stoddard Manufacturing Company Limited | Manufacturing | 1960 |
| Stollies Swim Academy Ltd | Education | 2024 |
| Stonar Close Residents Company Limited | Households | 1998 |
| Stonebridge Partners Ltd | Administrative & support | 2019 |
| Stonelands Court Management Company Limited | Households | 1986 |
| Stonely Properties (UK) Limited | Construction | 2013 |
| Stonewater Capital Limited | Real estate | 1997 |
| Stonewater Enterprises Limited | Financial services | 2015 |
| Stonewater Properties (Biggleswade) Limited | Real estate | 2020 |
| Stonewater Properties (Gravesend) Limited | Real estate | 2020 |
| Stonewater Properties (Stone Street) Limited | Real estate | 2020 |
| Stonewater Properties Group Limited | Financial services | 2020 |
| Stonewater Properties Limited | Real estate | 2012 |
| Storeroom Letchworth Ltd | Arts & recreation | 2021 |
| Story Worker Ltd | Arts & recreation | 2026 |
| Stotfold Management Company Limited | Households | 2007 |
| Strangely Calm Ltd | Information & communication | 2018 |
| Strata Coring Limited | Construction | 2001 |
| Strata Highways Limited | Extraterritorial | 2001 |
| Strategic Visioning Partners Ltd. | Professional & scientific | 1999 |
| Strategy Recruitment Solutions LLP | | 2007 |
| Street SIX Real Estate Ltd | Real estate | 2024 |
| Striving for Unity C.I.C. | Health & social care | 2021 |
| Stst Limited | Arts & recreation | 2019 |
| Studio 180 Ltd | Arts & recreation | 2013 |
| Studioxchange Limited | Wholesale & retail | 2008 |
| Suba Properties Ltd | Real estate | 2019 |
| Suba Properties RD Ltd | Real estate | 2020 |
| Sulantrix NO.2 Limited | Extraterritorial | 2025 |
| Summit Health and Fitness Ltd | Health & social care | 2025 |
| Sun Moon and Stars Flats Ltd | Households | 2021 |
| SUNNYJR21 Ltd | Health & social care | 2018 |
| Super Nice Coffee Shop Limited | Hospitality | 2025 |
| Superheat FGH Limited | Construction | 2008 |
| Supreme Tyres Ltd | Wholesale & retail | 2024 |
| SV Vapes Ltd | Professional & scientific | 2023 |
| Swanston Management Company Limited | Households | 1981 |
| Swift 5 Logistics Ltd | Transport & logistics | 2025 |
| Swift Cars Herts Ltd | Transport & logistics | 2025 |
| Swiftbridge Limited | Construction | 2006 |
| Swisstouch Electrics Ltd | Energy | 2021 |
| Sxproperty Ltd | Real estate | 2025 |
| Syed ALI Lettings Ltd | Real estate | 2020 |
| Synesco Limited | Professional & scientific | 2015 |
| Sypstraw Limited | Information & communication | 2017 |
| System Integration & Engineering Solutions Limited | Professional & scientific | 2006 |
| System Restore UK (Herts) Ltd | Wholesale & retail | 2021 |
| Systems Express Limited | Information & communication | 1988 |
| T & S White Decorating Limited | Construction | 2005 |
| T C Seamarks (Shot & Sand Blasting) Limited | Construction | 2005 |
| T D Property (Hitchin) Ltd | Households | 2020 |
| T H Smith Limited | Construction | 2025 |
| T R Maris Limited | Professional & scientific | 2013 |
| T&P Bespoke Sheet Metal Fabrications Ltd | Professional & scientific | 2020 |
| T.H.S Herts Ltd | Other services | 2023 |
| TAM Group Ltd | Manufacturing | 2024 |
| Tannon Holdings Ltd | Financial services | 2025 |
| TAP100 Ltd | Professional & scientific | 2018 |
| Task Business Group Ltd | Real estate | 2022 |
| Task Tech Group Ltd | Information & communication | 2022 |
| Taste of Ceylon Ltd | Manufacturing | 2024 |
| Tavie Limited | Information & communication | 2023 |
| Taylor Made Gardens Ltd | Administrative & support | 2023 |
| Taylored Plumbing Limited | Construction | 2006 |
| TBD Training & Consultancy Ltd | Professional & scientific | 2025 |
| TBI Analytics Limited | Information & communication | 2024 |
| TC (East) Limited | Administrative & support | 2024 |
| TC (East) NO2 Limited | Administrative & support | 2024 |