| Simon Bowen Painters and Decorators Limited | Construction | 2019 |
| Simon Dymond Limited | Professional & scientific | 2024 |
| Simon Edwards Flooring Limited | Construction | 2002 |
| Simon Hullin Developments Limited | Construction | 2003 |
| Simon Lear and Associates Limited | Information & communication | 2007 |
| Simon Tyre Services Ltd | Wholesale & retail | 2013 |
| Simplee Swim Ltd | Education | 2022 |
| Simpli Foods Ltd | Wholesale & retail | 2019 |
| Simplified Consultancy Ltd | Professional & scientific | 2022 |
| Simply Colors Limited | Wholesale & retail | 2008 |
| Simply Elegant Weddings & Events Ltd | Other services | 2019 |
| Simply Hemp Extracts Limited | Wholesale & retail | 2018 |
| Simply Repair Group Ltd | Other services | 2015 |
| Simply Safe Care Group Ltd | Health & social care | 2018 |
| Simply Safe Training Group Ltd | Education | 2020 |
| Simply V8 Motorcycles Ltd | Wholesale & retail | 2014 |
| Simply Vans Limited | Wholesale & retail | 2005 |
| Simpson Bartlett Limited | Health & social care | 2020 |
| Simpsons Estate Agents (Swansea) Limited | Real estate | 2010 |
| Simpsons Removals & Deliveries Ltd | Transport & logistics | 2019 |
| Sims & Jones Funeral Directors Limited | Other services | 2014 |
| Simswinggolf Ltd | Arts & recreation | 2026 |
| Singh Cabs Limited | Transport & logistics | 2025 |
| Singh Food Investment Ltd | Hospitality | 2018 |
| Singh's Motor Garage Ltd | Wholesale & retail | 2025 |
| Singing & Sound for Wellbeing Collective C.I.C. | Other services | 2026 |
| Singularity Healthtech Ltd | Health & social care | 2018 |
| Sinner Barbershop Limited | Other services | 2019 |
| Sinsir Limited | Professional & scientific | 2017 |
| Siop VAL CYF | Wholesale & retail | 2013 |
| Siora Ltd | Professional & scientific | 2025 |
| Sioreiddo Limited | Construction | 2026 |
| SIP Wealth Management Ltd | Administrative & support | 2003 |
| Siriol Lifestyle Limited | Agriculture & fishing | 2008 |
| Siriol Limited | Extraterritorial | 2012 |
| Sisu Sauna Ltd | Wholesale & retail | 2018 |
| SIT2SIP Ltd | Hospitality | 2026 |
| Sitara Jame Masjid Ltd | Other services | 2024 |
| Site & Facility Services Limited | Other services | 2024 |
| Site Geomatics Limited | Construction | 2018 |
| Site Safety Training Limited | Education | 2016 |
| Site Survey Plus Ltd | Professional & scientific | 2020 |
| Sitecare Services Ltd. | Construction | 2006 |
| Sitesmart Property Management Limited | Households | 1995 |
| Sivee Property Group Ltd | Real estate | 2024 |
| SIX 10 Enterprises Limited | Information & communication | 2006 |
| SIX Nations Rugby Limited | Information & communication | 1999 |
| Sixtythreesix Ltd | Professional & scientific | 2017 |
| SJ Aura Limited | Wholesale & retail | 2025 |
| SJ Bartram Ltd | Hospitality | 2024 |
| SJ Daniel Trading Limited | Hospitality | 2016 |
| SJ Davies Property Ltd | Real estate | 2019 |
| SJ Marketing Limited | Information & communication | 2018 |
| SJ Taylor Property Ltd | Real estate | 2026 |
| SJ Traders Limited | Wholesale & retail | 2022 |
| SJA Gibbons Limited | Arts & recreation | 2019 |
| SJA Martial Arts Limited | Education | 2019 |
| SJB Properties (South Wales) Ltd | Real estate | 2022 |
| SJC Electrical Contractors Ltd | Construction | 2023 |
| SJC Electrical Services Bristol Ltd | Construction | 2010 |
| SJG Scrap and Waste Ltd | Water & waste | 2025 |
| SJH Couriers Ltd | Transport & logistics | 2016 |
| SJH Hospitality Limited | Hospitality | 2023 |
| Sjht Transport Ltd | Transport & logistics | 2020 |
| Sjjr Properties Ltd | Real estate | 2025 |
| SJS Construction Services Limited | Construction | 2016 |
| SJS Live Limited | Hospitality | 2025 |
| SJT (Swansea) Ltd | Hospitality | 2022 |
| Sjvg Holdings Ltd | Real estate | 2018 |
| Sjvg Properties Ltd | Real estate | 2018 |
| SK Fish Bar Ltd | Hospitality | 2024 |
| SK Friends Ltd | Wholesale & retail | 2020 |
| SK Hotels and Resorts UK Ltd | Hospitality | 2017 |
| Skaizone Global Property Services Ltd | Real estate | 2022 |
| Sketty Barbers Limited | Other services | 2024 |
| Sketty Express Ltd | Wholesale & retail | 2026 |
| Sketty GAS Ltd | Construction | 2024 |
| Sketty Ground Rents Limited | Real estate | 2021 |
| Sketty Management Company Limited | Households | 2014 |
| Sketty Marketing Solutions Ltd | Professional & scientific | 2021 |
| Sketty Packed Meals Limited | Hospitality | 2024 |
| Sketty Property Company Limited | Arts & recreation | 1962 |
| Sketty Tech LAB Ltd | Other services | 2026 |
| Skidz Karting Limited | Arts & recreation | 2010 |
| Skilled Technologies Ltd | Information & communication | 2026 |
| Skillzify Impact CIC | Information & communication | 2024 |
| Skin Care Cymru | Health & social care | 2007 |
| Skinners Arms Ltd | Hospitality | 2023 |
| Skinni V Ltd | Wholesale & retail | 2019 |
| SKL Vehicle Transport Ltd | Transport & logistics | 2022 |
| SKM Group Limited | Manufacturing | 2017 |
| SKM Medical Limited | Health & social care | 2012 |
| Skoda Direct (UK) Limited | Wholesale & retail | 2001 |
| Skoodle Ltd | Education | 2025 |
| SKR Electrical Solutions Ltd | Construction | 2022 |
| Skull Drive Ltd | Wholesale & retail | 2020 |
| Sky Grab Ltd | Professional & scientific | 2017 |
| Sky High Consulting Ltd | Professional & scientific | 2026 |
| Skybluecreative Ltd | Information & communication | 2020 |
| Skyfall Building Services Ltd | Construction | 2023 |