| Seaward Electrical Ltd | Construction | 2026 |
| Seawinds Consultants Limited | Administrative & support | 2012 |
| Sebastian & ST. James Financial Advisers (UK) Limited | Financial services | 2014 |
| Security Made Easier Holdings Limited | Administrative & support | 2018 |
| Semo Limited | Hospitality | 2007 |
| Sentinel Business Consultants Ltd | Professional & scientific | 2021 |
| Seren Financial Advisers Ltd | Financial services | 2012 |
| Serverless Events Limited | Administrative & support | 2018 |
| Sevco 5111 Limited | Real estate | 2013 |
| Severn Technical Services Ltd | Manufacturing | 2013 |
| Severnside Property Management Ltd | Real estate | 2021 |
| Severnside TDC Fabrication Ltd | Manufacturing | 2022 |
| Severnside Transportation Data Collection Ltd | Information & communication | 2018 |
| SG Aircraft Maintenance Ltd | Manufacturing | 2018 |
| SH Weld & Installation Ltd | Manufacturing | 2025 |
| Shakti Garages Limited | Wholesale & retail | 2025 |
| Sharb Limited | Hospitality | 2017 |
| Shelley Manor Limited | Health & social care | 2010 |
| Shier Coatings Ltd | Construction | 2022 |
| Shine Cardiff Road Hand Car Wash Ltd | Administrative & support | 2025 |
| Shinrai Consulting Ltd | Professional & scientific | 2025 |
| Shudehill Properties Limited | Professional & scientific | 1955 |
| SI Belle Property's Ltd | Real estate | 2022 |
| SI4 Security Limited | Administrative & support | 2024 |
| Sian Beauty Limited | Other services | 2025 |
| Sian Grigoriou Limited | Information & communication | 2012 |
| Siddhi PVT Ltd | Financial services | 2024 |
| Sierra One Consultancy Ltd | Administrative & support | 2024 |
| Silver Campers Ltd | Manufacturing | 2023 |
| Simon Rowles Financial Planning Limited | Financial services | 2013 |
| Simon Walker (Transport) Limited | Transport & logistics | 2007 |
| Simon's K9 Care Ltd | Agriculture & fishing | 2023 |
| Simply Clean CCS Limited | Administrative & support | 2024 |
| Simpson's Power Services Limited | Construction | 2023 |
| Sinclair Carpentry Ltd | Manufacturing | 2025 |
| Siris Properties Limited | Real estate | 2025 |
| Sister's Designs Limited | Wholesale & retail | 2025 |
| SJ Holdings (Barry) Ltd | Hospitality | 2025 |
| SJ Tech Ltd | Information & communication | 2026 |
| Skin by KF Limited | Other services | 2023 |
| Skincare by Gemma Ltd | Wholesale & retail | 2023 |
| Skomer Road Pizza Limited | Hospitality | 2025 |
| Skomer Road Stores Ltd | Wholesale & retail | 2024 |
| SKS Safety Limited | Administrative & support | 2009 |
| SKS Safety Services Limited | Administrative & support | 2024 |
| Sky Spa Wellness Centre Ltd | Other services | 2020 |
| Skycare Tours and Travels Limited | Administrative & support | 2022 |
| Skyferry Aviation Services Ltd | Professional & scientific | 2026 |
| Skypermit Ltd | Administrative & support | 2025 |
| SL Building & Plastering Ltd | Construction | 2025 |
| Slimming in Barry Ltd | Arts & recreation | 2016 |
| Slock Electrical Limited | Construction | 2016 |
| SM Pitcock Flooring Services Ltd | Construction | 2017 |
| SMA & DK PVT Ltd | Wholesale & retail | 2025 |
| Smashed Apple Limited | Other services | 2019 |
| SMC Coaching Ltd | Arts & recreation | 2025 |
| SMK Building and Maintenance Limited | Construction | 2002 |
| SMK Renovate Limited | Construction | 2021 |
| Smooth Decorators Ltd | Construction | 2024 |
| Smooth Ride Tyre and Auto Centres Ltd | Wholesale & retail | 2011 |
| Smooth Starts Plus Limited | Health & social care | 2013 |
| SMP Investing Ltd | Real estate | 2026 |
| SMS Cleaning Ltd | Administrative & support | 2025 |
| SMS Maintenance Ltd | Administrative & support | 2026 |
| Smugglers Cove Barry Island Limited | Arts & recreation | 2010 |
| Snowcat Clothing Ltd | Wholesale & retail | 2020 |
| Sobytes Limited | Information & communication | 2012 |
| Sofia Magazine Ltd | Information & communication | 2025 |
| Sofias Souvenirs Limited | Wholesale & retail | 2022 |
| Solar 2 Fuel Ltd | Manufacturing | 2021 |
| Solid Yields Ltd | Real estate | 2025 |
| Solina House Ltd | Administrative & support | 2025 |
| Solomons Garden Limited | Arts & recreation | 2018 |
| Soltherm EWI (Wales) Limited | Construction | 2013 |
| Some Good Company Limited | Wholesale & retail | 2020 |
| Soon Ltd | Real estate | 2019 |
| Soran Barbers Ltd | Other services | 2024 |
| Soraya’S Cocktails and Desserts Ltd | Hospitality | 2021 |
| Sorelle Hub Limited | Other services | 2026 |
| Sorenna Hair and Beauty Wholesale Ltd | Wholesale & retail | 2026 |
| Soro Properties Ltd | Real estate | 2025 |
| Sotc Communications Ltd | Professional & scientific | 2021 |
| Souls Awakening Ltd | Other services | 2020 |
| Sound Saver Limited | Wholesale & retail | 2023 |
| South Wales Liquidations Ltd | Wholesale & retail | 2025 |
| South Wales Martial Arts Academy CIC | Education | 2022 |
| South Wales Stairlifts Ltd | Manufacturing | 2011 |
| South Wales Time Recorder (Sales and Service) Limited | Manufacturing | 1979 |
| Space B Tiling Limited | Construction | 2019 |
| Sparkles Cleaning Services Wales & West Limited | Administrative & support | 2018 |
| Spearmaker Ltd | Extraterritorial | 2023 |
| Specialist Coatings (Smith & SON) Ltd. | Wholesale & retail | 1992 |
| Splash Swim School Ltd | Arts & recreation | 2011 |
| Splashndough Limited | Hospitality | 2023 |
| Springspa Court Management Company Limited | Households | 1974 |
| Spry Enterprises Ltd | Other services | 2021 |
| Spud ME Cardiff Ltd | Hospitality | 2025 |
| Spudmebarry Ltd | Hospitality | 2024 |
| Spyca Limited | Real estate | 2012 |
| SQE Limited | Administrative & support | 2004 |