| J. Homes Ltd | Construction | 1995 |
| J. Star Ltd | Wholesale & retail | 2025 |
| J. V. Davies Painters and Decorators Limited | Construction | 2005 |
| J.A Plumbing & Bathrooms Limited | Construction | 2023 |
| J.D Heating and GAS Engineers Ltd | Construction | 2019 |
| J.D. Power UK Ltd | Professional & scientific | 2022 |
| J.D.J Carpentry Ltd | Manufacturing | 2025 |
| J.E.F. Scaffolding Limited | Construction | 1974 |
| J.F. Cahill Limited | Construction | 2006 |
| J.J and J.L LEE Limited | Other services | 2015 |
| J.J.L. Care Limited | Health & social care | 2008 |
| J.K's Automotive Ltd | Wholesale & retail | 2024 |
| J.K. Electrical Services Ltd | Construction | 2003 |
| J.L.Biggs Limited | Professional & scientific | 2016 |
| J.M. Fabweld Limited | Manufacturing | 1968 |
| J.Murphy Future Designs Limited | Professional & scientific | 2019 |
| J.Oddy Ltd | Wholesale & retail | 2025 |
| J.R. Davies Fuels Limited | Wholesale & retail | 1982 |
| J.S. Davies & SON 2011 Limited | Construction | 2011 |
| J.S.V. Stainless Steel Fabrications Limited | Manufacturing | 2000 |
| J2 Developments Ltd | Construction | 2024 |
| J3 Electrical Services Limited | Construction | 2016 |
| J60 Limited | Information & communication | 2012 |
| JA Management 2024 Limited | Professional & scientific | 2024 |
| Jaag Fire & Flood Ltd | Administrative & support | 2024 |
| Jaas Healthcare Limited | Health & social care | 2024 |
| Jabro Games Limited | Manufacturing | 2006 |
| Jabro Harnesses Limited | Manufacturing | 2008 |
| Jabus Limited | Information & communication | 2025 |
| JAC Freeholders Limited | Real estate | 2017 |
| Jack & Danny TV Limited | Information & communication | 2005 |
| Jack IT UP Ltd | Hospitality | 2025 |
| Jack Murphys Swansea Ltd | Hospitality | 2019 |
| Jack of Clubs Holdings Limited | Financial services | 2021 |
| Jack of Clubs Ltd | Hospitality | 2017 |
| Jack Sullivan (Properties) Limited | Wholesale & retail | 1997 |
| Jack's Ice Creams Ltd | Wholesale & retail | 2022 |
| Jack's Veterinary Ltd | Professional & scientific | 2021 |
| JACK47MEDIA Ltd | Other services | 2025 |
| Jackate Limited | Real estate | 2016 |
| Jacked Potatoes Limited | Hospitality | 2025 |
| Jackies Cleaning Services Ltd | Administrative & support | 2024 |
| Jacks Digital Ltd. | Information & communication | 2017 |
| Jacob & Associates Ltd | Hospitality | 2004 |
| Jacob Agri Ltd | Agriculture & fishing | 2024 |
| Jacob Holt Investments Ltd | Real estate | 2017 |
| Jacob Lewis Trees Limited | Agriculture & fishing | 2015 |
| Jacob Property Management Ltd | Real estate | 2026 |
| Jacobus Shipping Limited | Administrative & support | 2021 |
| Jacpa Ltd | Education | 2024 |
| Jad's Gymnasiums Limited | Arts & recreation | 1981 |
| Jadeed Limited | Construction | 2018 |
| Jadw Sales Ltd | Wholesale & retail | 2025 |
| Jaffgaff Limited | Real estate | 2018 |
| JAG Funding Solutions Ltd | Financial services | 2023 |
| JAG Group Ltd | Real estate | 2018 |
| JAG PD Ltd | Construction | 2019 |
| JAG Property Investments Limited | Real estate | 2016 |
| Jake Joyce Chiropractic Limited | Extraterritorial | 2017 |
| Jake Lloyd Timber Limited | Transport & logistics | 2023 |
| Jakupi Construction Ltd | Construction | 2023 |
| JAM Co Holdings Ltd | Financial services | 2026 |
| JAM CO1 Ltd | Construction | 2026 |
| JAM Innovations Limited | Administrative & support | 2023 |
| JAM Morris Marketing Ltd | Professional & scientific | 2025 |
| JAM Portfolio Ltd | Real estate | 2013 |
| JAM TAN Foods Ltd | Manufacturing | 2023 |
| JAM Today Marketing Ltd | Professional & scientific | 2023 |
| Jamaa Telecom Limited | Information & communication | 2016 |
| Jambob Property Limited | Real estate | 2023 |
| Jameela Healthcare Swansea PVT Ltd | Health & social care | 2018 |
| James & Collier Developments Ltd | Real estate | 2021 |
| James & Jones Properties Ltd | Real estate | 2024 |
| James & Uzzell Limited | Professional & scientific | 1998 |
| James & Uzzell Properties Limited | Real estate | 2012 |
| James Andrew Thomas Ltd | Health & social care | 2026 |
| James Contract Services Ltd | Manufacturing | 2012 |
| James Court Holdings Limited | Other services | 2016 |
| James Developments (Swansea) Ltd | Real estate | 2024 |
| James Drylining Ltd | Construction | 2025 |
| James Evans Limited | Construction | 2014 |
| James Good Limited | Professional & scientific | 2002 |
| James Hook Limited | Other services | 2007 |
| James L Dean Limited | Professional & scientific | 2018 |
| James Lewis Plumbing and Heating Services Ltd | Other services | 2023 |
| James MO Design Limited | Professional & scientific | 2011 |
| James Norbury Ltd | Arts & recreation | 2021 |
| James Thomas Builders Limited | Construction | 2012 |
| James Westacott Limited | Professional & scientific | 2015 |
| Jamesbelle Grace Properties Ltd | Real estate | 2024 |
| Jamesmillersolutions Ltd | Manufacturing | 2020 |
| Jamie Donaldson Limited | Arts & recreation | 2013 |
| Jamie George Fitness Ltd | Arts & recreation | 2024 |
| Jamie Mcintyre Ltd | Financial services | 2013 |
| Jamie Mcintyre Properties Limited | Real estate | 2025 |
| Jamie Short Ltd | Wholesale & retail | 2014 |
| Jamie Woolcock Limited | Administrative & support | 2014 |
| Jammin' Catering Limited | Hospitality | 2022 |
| Jammin' Catering OPS Limited | Hospitality | 2024 |
| Jamshid-E JAM Supermarket Ltd | Wholesale & retail | 2025 |