| Simon Jones Cars Limited | Wholesale & retail | 2006 |
| Simplitax (Wales) Limited | Professional & scientific | 2003 |
| Simply Indian Merthyr Limited | Hospitality | 2025 |
| Sindherfoods Limited | Wholesale & retail | 2011 |
| Siteserve Construction Ltd | Construction | 2015 |
| Siva Cabs Limited | Transport & logistics | 2024 |
| Sixty Sixty Coaches Limited | Transport & logistics | 2018 |
| SJ Engineered Ltd | Wholesale & retail | 2022 |
| SJD Property Group Ltd | Real estate | 2023 |
| SJW Carpentry & Building Limited | Construction | 2015 |
| Skar Commercial Storage Limited | Real estate | 2019 |
| Skar Scaffolding Limited | Construction | 2005 |
| Sleepy Stays Limited | Real estate | 2021 |
| Small Red HAT Limited | Hospitality | 2017 |
| Small Training Solutions Limited | Professional & scientific | 2019 |
| Smart Mechanical Engineering Ltd | Construction | 2023 |
| Soar Variation Arts C.I.C. | Arts & recreation | 2025 |
| Solar Clean Services Ltd | Administrative & support | 2026 |
| Solarfit Energy Ltd | Construction | 2025 |
| Soldi Films Ltd | Information & communication | 2021 |
| Sole Mate Running Ltd | Wholesale & retail | 2022 |
| Sollace UK Ltd | Administrative & support | 2025 |
| South Wales Single PLY Roofing Limited | Construction | 2013 |
| Southview Property Investments Ltd | Real estate | 2017 |
| Spark Programmes Limited | Other services | 2021 |
| Sparkles Bar Ltd | Hospitality | 2024 |
| Spartan Tools Limited | Wholesale & retail | 2003 |
| Spensum Coffee Ltd | Manufacturing | 2022 |
| Springfield Limited Company Ltd | Health & social care | 2012 |
| SS Software and Solutions Limited | Information & communication | 2025 |
| SSR Reddys Limited | Wholesale & retail | 2023 |
| SSR Reddys Property Limited | Real estate | 2024 |
| SSW Ltd | Construction | 2017 |
| ST Tydfil Fashion Nails Spa Ltd | Other services | 2024 |
| Star Rail Resourcing Limited | Construction | 2016 |
| Star's Studio UK Limited | Information & communication | 2025 |
| Stargrove Resources Limited | Information & communication | 2001 |
| Station Terrace 1967 Limited | Extraterritorial | 2015 |
| Stephen Harris Limited | Construction | 2016 |
| Stephen Shane Builders Limited | Construction | 2013 |
| Stephen Shane Properties Limited | Real estate | 2024 |
| Stephens & George Commercial Limited | Extraterritorial | 1988 |
| Stephens and George Limited | Manufacturing | 1946 |
| Stephens and George Magazines Limited | Extraterritorial | 1994 |
| Sterilesafe Solutions Limited | Administrative & support | 2025 |
| Sterility Assurance Limited | Professional & scientific | 2023 |
| Steve Williams Fruit and VEG Ltd | Wholesale & retail | 2026 |
| Steve's Domestics Limited | Other services | 2012 |
| Stoae Limited | Hospitality | 2025 |
| Stoic Handbook Ltd | Information & communication | 2025 |
| Stone Archie Limited | Administrative & support | 2019 |
| Stonebridge Properties and Developments Limited | Real estate | 2025 |
| Storage Centre (Wales) Ltd | Administrative & support | 2011 |
| Strait & Narrow Marine Ltd | Transport & logistics | 2024 |
| Structures & Civils Solutions Ltd | Construction | 2024 |
| Studios at Glenthorne Limited | Real estate | 2016 |
| Style CUT N Comb Ltd | Other services | 2026 |
| SUB 5 Car Club Ltd | Administrative & support | 2023 |
| Sullivans Auctions Limited | Other services | 2016 |
| Sun 68 W Ltd | Hospitality | 2024 |
| Sunrise Haven Family Support C.I.C. | Health & social care | 2026 |
| Supac Limited | Wholesale & retail | 1952 |
| Superstar Industries UK Ltd | Arts & recreation | 2021 |
| Supplement Paradise Ltd | Wholesale & retail | 2019 |
| Supportability Ltd | Education | 2026 |
| Sussex Fencing Services Limited | Construction | 2006 |
| Sutty Slabing Limited | Construction | 2017 |
| SVR Handyman Services Limited | Manufacturing | 2020 |
| SW Gallus Limited | Wholesale & retail | 2026 |
| Switched on Solutions Electrical Services Limited | Construction | 2019 |
| SYS Properties Ltd | Construction | 2024 |
| T&D Stores Ltd | Wholesale & retail | 2023 |
| T&W Property Haven Ltd | Real estate | 2025 |
| T.P. Griffiths & Sons (Merthyr Tydfil) Ltd | Other services | 2014 |
| T3 Rail and Civils Ltd | Construction | 2025 |
| TAF Electrical Group Ltd | Construction | 2026 |
| Taff Group Property Services Ltd | Construction | 2026 |
| Taff’S Taxis Limited | Transport & logistics | 2025 |
| Tahawks Ltd | Information & communication | 2025 |
| TAJ Catering Limited | Hospitality | 2018 |
| Talking Heads Social Work Consultation Ltd | Health & social care | 2021 |
| Talu Group Limited | Information & communication | 2024 |
| TAN Bomb Limited | Wholesale & retail | 2026 |
| Tan-Ex Holdings Limited | Other services | 2019 |
| Tan-Ex Investments Limited | Real estate | 2023 |
| Tan-Ex Medical Ltd | Health & social care | 2019 |
| Tarianau Security Ltd | Administrative & support | 2021 |
| Taurus RBM Limited | Wholesale & retail | 2015 |
| Taylors Construction Services Ltd | Construction | 2021 |
| Taylors Forge Group Limited | Financial services | 2026 |
| TCS Homecare Limited | Health & social care | 2021 |
| Technical Environmental Services Limited | Extraterritorial | 2001 |
| Technical Services Consult Ltd | Construction | 2018 |
| Teddy Evans Photography Ltd | Professional & scientific | 2019 |
| TEE2SUGARS Ltd | Professional & scientific | 2025 |
| Tele-Response Limited | Information & communication | 2003 |
| Tenneco-Walker(U.K.)Limited | Manufacturing | 1970 |
| Terra Foods Ltd | Hospitality | 2018 |
| Teulu Carpentry and Landscaping Ltd | Manufacturing | 2020 |
| That's Tidy Limited | Information & communication | 2016 |