| ISW Fit-Out Limited | Construction | 2019 |
| ISW Reed Ltd | Health & social care | 2023 |
| Items First Limited | Wholesale & retail | 2016 |
| Ivanova Publications Ltd | Information & communication | 2023 |
| Ivanovi Properties Ltd | Real estate | 2024 |
| IVD Electric Ltd. | Manufacturing | 2020 |
| Iveson Scaffolding Ltd | Construction | 2024 |
| Ivey Patisseries Limited | Manufacturing | 2019 |
| J & F Homes Ltd | Real estate | 2024 |
| J & J Property Developers and Investments Ltd | Households | 2024 |
| J a H Bricklayers Limited | Construction | 2014 |
| J Coombes Ltd | Real estate | 2024 |
| J E J Construction Limited | Construction | 2025 |
| J E Parsons Limited | Construction | 2019 |
| J F Hutchings Ltd | Wholesale & retail | 2012 |
| J Jones Consultancy Limited | Information & communication | 2008 |
| J L Preece Flooring Ltd | Wholesale & retail | 2019 |
| J M Dimonaco Ltd | Information & communication | 2022 |
| J M G Plumbing Limited | Construction | 2012 |
| J M Training & Product Academy Limited | Education | 2024 |
| J Price Limited | Health & social care | 2003 |
| J R Davies Engineering Services Limited | Construction | 2016 |
| J Simpson Transport Limited | Transport & logistics | 2024 |
| J Slade Limited | Manufacturing | 2023 |
| J Talbot Groundwork Ltd | Construction | 2011 |
| J W Joinery (South Wales) Limited | Construction | 2014 |
| J&K Global Limited | Wholesale & retail | 2008 |
| J&R Carpentry Ltd | Manufacturing | 2025 |
| J.D.W. Gambarini Limited | Manufacturing | 2013 |
| J.Taylor(Cardiff)Limited | Real estate | 1950 |
| J.W. Dean (Tonypandy) Limited | Wholesale & retail | 1979 |
| J2 Contract Flooring Ltd | Construction | 2025 |
| JA Comforts Ltd | Extraterritorial | 2018 |
| Jacks Wood Avenue 1979 Limited | Extraterritorial | 2014 |
| JAM and Rice Properties Ltd | Real estate | 2024 |
| James & Rowe Drylining Ltd | Construction | 2022 |
| James DE Frias Limited | Professional & scientific | 2001 |
| Jamestone Properties Limited | Real estate | 2002 |
| Janet Better Healthcare Limited | Health & social care | 2022 |
| Janwei Northern Food Ltd | Hospitality | 2025 |
| Jardenaje Limited | Real estate | 2022 |
| Jarred J Couriers Ltd | Transport & logistics | 2026 |
| Jarredj Logistics Limited | Transport & logistics | 2025 |
| Jason Edwards Travel Limited | Transport & logistics | 2023 |
| Jaycon Solutions Ltd | Professional & scientific | 2013 |
| Jbcw Properties Ltd | Real estate | 2024 |
| JC Equine Services Ltd | Agriculture & fishing | 2025 |
| JCW Design & Build Limited | Construction | 2020 |
| JEM Industries Ltd | Construction | 2024 |
| JEM Interiors Specialists Ltd | Construction | 2019 |
| Jenkins & Batchelor Construction Limited | Construction | 2019 |
| Jenkins and Bailey Ltd | Real estate | 2025 |
| Jenkins Electrical and Renewables Limited | Construction | 2023 |
| Jenkins Products Limited | Wholesale & retail | 2018 |
| Jenleigh Foods Limited | Wholesale & retail | 2008 |
| Jerry Trading Group Ltd | Wholesale & retail | 2025 |
| Jessica Moule Hair Limited | Other services | 2021 |
| Jevan Investments Ltd | Real estate | 2018 |
| JG Construction & Paving Limited | Construction | 2023 |
| JH & JH Property Limited | Real estate | 2018 |
| Jinming Tech Ltd | Wholesale & retail | 2024 |
| Jinvoo Smart Tech Ltd | Wholesale & retail | 2025 |
| Jitz Fest Limited | Other services | 2024 |
| Jiujinhe Limited | Wholesale & retail | 2023 |
| JJ & S Plumbing and Heating Ltd | Construction | 2024 |
| JJ Morgan Consulting Limited | Real estate | 2017 |
| JJ Prime 2 Properties Ltd | Real estate | 2024 |
| JJ Prime Properties Ltd | Real estate | 2024 |
| JJW Technologies Limited | Professional & scientific | 2024 |
| JLE Electrical Ltd | Construction | 2025 |
| JLG Media Ltd. | Professional & scientific | 2024 |
| JLK Plumbing and Bathroom Services Limited | Construction | 2026 |
| JLL Property Ltd | Real estate | 2025 |
| Jo-Bb Ltd | Health & social care | 2023 |
| Joanj Group Limited Ltd | Transport & logistics | 2026 |
| Jocbservices Limited | Wholesale & retail | 2021 |
| JOE Gambarini Ltd | Financial services | 2019 |
| John Curley Scaffolding (Wales) Limited | Construction | 2013 |
| John F. Hutchings Limited | Wholesale & retail | 1976 |
| John Henry Local Ltd | Real estate | 2017 |
| John Mcdonald Golf Limited | Wholesale & retail | 2015 |
| Jones Building and Contracting Ltd | Construction | 2026 |
| Jones the Numbers CYF. | Professional & scientific | 2025 |
| Jordan Chan Catering Ltd | Hospitality | 2019 |
| Jordan Sheehy Videography Ltd | Information & communication | 2026 |
| Joshua Pritchard Services Ltd | Construction | 2021 |
| Joyful Creations Ltd | Wholesale & retail | 2024 |
| JP Interior Systems Limited | Construction | 2021 |
| Jp&Lp Properties Ltd | Households | 2025 |
| JPL Cars Limited | Other services | 2023 |
| Jrbd Company Support Ltd | Professional & scientific | 2018 |
| Jrbd Ltd | Administrative & support | 2015 |
| Jrbdmicro Ltd | Professional & scientific | 2015 |
| JRS Business Services Limited | Professional & scientific | 2018 |
| Jsecurity Limited | Public administration | 2019 |
| JT Properties (South Wales) Limited | Households | 2013 |
| Jtec Machinery Ltd | Mining & quarrying | 2019 |
| Jude Prime Estates PVT Ltd | Real estate | 2025 |
| Julia Denny Limited | Professional & scientific | 2013 |
| Julie Sakkas Ltd | Professional & scientific | 2012 |