| Silver Fern Therapy Ltd | Health & social care | 2013 |
| Silversure Property Services Ltd | Construction | 2024 |
| Silvey Conservation Heritage Roofing & Building Limited | Construction | 2013 |
| Simple Caterer Ltd | Hospitality | 2018 |
| Simply Devine Limited | Professional & scientific | 2018 |
| Simrin Rose Properties Ltd | Financial services | 2025 |
| Singlemost Ltd | Information & communication | 2006 |
| Siris Thai Spa Limited | Other services | 2024 |
| Sister's Holdings Limited | Financial services | 2021 |
| SJR Surveying Ltd | Professional & scientific | 2022 |
| Sjwh Holdings Limited | Professional & scientific | 2021 |
| SKI Marketing Limited | Administrative & support | 2013 |
| Skyasset Limited | Hospitality | 2001 |
| SLE Events Ltd | Arts & recreation | 2022 |
| Sleepy Hollow Property Ltd | Real estate | 2025 |
| Smart Home Real Estate Ltd | Real estate | 2024 |
| Smart House Archi.Tech Limited | Construction | 2019 |
| Smarter Solution Design Limited | Professional & scientific | 2018 |
| SMD Ventilation Limited | Construction | 2020 |
| Smee Development Limited | Construction | 2025 |
| Smith Group Ltd | Other services | 2025 |
| Smith Well-Being Index (SWI) Limited | Professional & scientific | 2025 |
| SMJ Professional Services Limited | Professional & scientific | 2000 |
| SMM Advisory Ltd | Professional & scientific | 2026 |
| Smpd Limited | Real estate | 2013 |
| SMR Training Limited | Education | 2020 |
| Snells Office Supplies Limited | Wholesale & retail | 2021 |
| SO Stylish Soph Ltd | Professional & scientific | 2026 |
| Soar Construction Limited | Construction | 2024 |
| Soar Property Limited | Real estate | 2025 |
| SOL Products Limited | Manufacturing | 2003 |
| Soltys Brewster Ecology Limited | Professional & scientific | 2006 |
| Soltys Brewster Limited | Extraterritorial | 2011 |
| Soltys Portfolio Limited | Financial services | 2017 |
| Solvingmaze Ltd | Information & communication | 2017 |
| Sophia Mansions Penarth Management Limited | Households | 2024 |
| Soul SET Construction Ltd | Manufacturing | 2022 |
| Soulindata Ltd | Information & communication | 2023 |
| South Wales Auto Limited | Wholesale & retail | 2017 |
| South Wales BUS and Van Centre Limited | Wholesale & retail | 2025 |
| South Wales Windscreens Limited | Wholesale & retail | 2025 |
| Space Experiments Limited | Professional & scientific | 2017 |
| Spaced OUT Store Ltd | Wholesale & retail | 2023 |
| Sparkle Cleaning & Property Maintenance Ltd | Professional & scientific | 2019 |
| Spartan Flooring Limited | Wholesale & retail | 2019 |
| Specialist Paint Effect Company Limited | Administrative & support | 2006 |
| Specialist Rescue Solutions Ltd | Health & social care | 2025 |
| Spectrum Holdco Ltd | Financial services | 2026 |
| Spectrum Operations Ltd | Administrative & support | 2026 |
| Speed Estate Management Limited | Real estate | 2022 |
| Speed Ventures Limited | Arts & recreation | 2019 |
| Spill Training UK Ltd | Education | 2024 |
| Spirit Commercial Services Limited | Professional & scientific | 2021 |
| Spirit Public Sector Ltd | Professional & scientific | 2011 |
| Spiritus Limited | Other services | 2005 |
| SPJ Carpentry Services Ltd | Manufacturing | 2024 |
| SPL TA Holdings Limited | Real estate | 2017 |
| Sport Travel 4U Ltd | Arts & recreation | 2026 |
| Spring Garden (Penarth) Limited | Hospitality | 2021 |
| SR & L Tandem Ltd | Real estate | 2016 |
| SS Data Center Solutions Limited | Professional & scientific | 2025 |
| ST Aubin Nurseries (Roath) Limited | Health & social care | 2019 |
| ST David's Court Management Company Limited | Households | 1994 |
| ST Peter's OUT of School Care Limited | Health & social care | 2020 |
| ST Thomas (Swansea) Management Limited | Households | 2009 |
| Stamford Optical Limited | Wholesale & retail | 2023 |
| Stanacha Limited | Administrative & support | 2016 |
| Stanwell Court Management Company Limited | Households | 1976 |
| Stanwell Road Management Company Limited | Households | 2016 |
| Starline Cleaning Ltd | Administrative & support | 2025 |
| Start UP Wales Limited | Extraterritorial | 2009 |
| Startmutual Property Management Limited | Households | 1991 |
| Station Court (Dinas Powys) Residents Association Limited | Professional & scientific | 2004 |
| STAY22 Ltd | Real estate | 2025 |
| Staybridge Ltd | Real estate | 2025 |
| Stella Consulting Limited | Professional & scientific | 2023 |
| Stem Untapped CIC | Information & communication | 2021 |
| Stephen Cannon Consultancy Ltd | Health & social care | 2021 |
| Steptree Consultants Limited | Administrative & support | 2017 |
| Stocktogodotco Limited | Real estate | 2025 |
| Stonecity Creative Limited | Information & communication | 2025 |
| Stonegrove Property Developments Ltd | Construction | 2025 |
| Storm Architecture Ltd | Professional & scientific | 2026 |
| Strand Developments (Swansea) Limited | Real estate | 2007 |
| Strategy and Solutions Limited | Professional & scientific | 2010 |
| Striking Attitudes Limited | Arts & recreation | 2000 |
| Studio Lotte Ltd | Professional & scientific | 2011 |
| Studiosixtyfive Limited | Households | 2003 |
| Sturdy Plumbing Company Limited | Construction | 1968 |
| Suburban Taps Ltd | Hospitality | 2018 |
| Sully Clubhouse Limited | Hospitality | 2024 |
| Sully Marine Centre Limited | Administrative & support | 2005 |
| Sully Motors Ltd | Wholesale & retail | 2020 |
| Sullylocal Limited | Wholesale & retail | 2025 |
| Summit Interior Systems Ltd | Construction | 2026 |
| Summora Ltd | Wholesale & retail | 2025 |
| Sumu Construction Ltd | Construction | 2023 |
| Sumu Homes Ltd | Real estate | 2023 |
| Sunscape Systems Limited | Construction | 2012 |
| Superguns Ltd | Wholesale & retail | 2022 |