| J Lewis Building Solutions Limited | Manufacturing | 2022 |
| J R Industries Limited | Manufacturing | 1970 |
| J&A Hughes Properties LLP | | 2026 |
| J&C Enterprises Ltd | Hospitality | 2023 |
| J. C. Welding Supplies Limited | Wholesale & retail | 1996 |
| J. Manny Limited | Manufacturing | 2003 |
| J. V. Rowe & Sons Limited | Extraterritorial | 1998 |
| J.& M. Cook (Builders) Limited | Construction | 1988 |
| J.C Tailors Ltd | Professional & scientific | 2019 |
| J.Pesci & Sons Limited | Water & waste | 1953 |
| J2K Consulting Limited | Administrative & support | 2021 |
| JAC Electrical (Cardiff) Ltd | Construction | 2022 |
| Jade Consulting Limited | Real estate | 2014 |
| Jagera Holdings Ltd | Households | 2024 |
| Jalop Limited | Real estate | 2016 |
| Jalt Properties Limited | Real estate | 2025 |
| Jamas Properties Limited | Real estate | 1999 |
| James Crypto Bull Ltd | Extraterritorial | 2016 |
| James Executive Travel Ltd | Transport & logistics | 2015 |
| Jameslife Caring Company Limited | Health & social care | 2023 |
| Jasmin Taylor Training Ltd | Information & communication | 2020 |
| Jason Bellamy Flooring Limited | Wholesale & retail | 2020 |
| Java Properties (South Wales) Ltd | Real estate | 2022 |
| JB Electrical (Wales) Ltd | Construction | 2014 |
| JB Legal Consultant Ltd | Professional & scientific | 2017 |
| Jcswales Limited | Transport & logistics | 2020 |
| JDE (Evans) Limited | Financial services | 2024 |
| JDS Detailing Ltd | Administrative & support | 2021 |
| Jefferies Properties Ltd | Real estate | 2021 |
| JEL Property Limited | Real estate | 2020 |
| Jermaine Harris Ltd | Education | 2016 |
| Jessica and the Moon Ltd | Other services | 2024 |
| JET Materials Limited | Wholesale & retail | 2010 |
| Jiffy JET Ltd | Administrative & support | 2018 |
| Jilted Industries Limited | Financial services | 2025 |
| Jimmy Angove PD Ltd | Financial services | 2025 |
| Jjbs Bookkeeping Solutions Ltd | Professional & scientific | 2015 |
| JK Raven Properties Ltd | Real estate | 2019 |
| Jksdistributions Ltd | Wholesale & retail | 2024 |
| JM Fire & Security Systems Ltd | Administrative & support | 2021 |
| JM Locum Ltd | Wholesale & retail | 2021 |
| Jmdbt Ltd | Professional & scientific | 2023 |
| JME Building & Plastering Limited | Administrative & support | 2018 |
| JMG Consulting and Investments Limited | Construction | 2025 |
| JMR Global Supplies Ltd | Wholesale & retail | 2019 |
| JMT Plumbing and Heating Ltd | Construction | 2022 |
| Jodie Hillier Ltd | Health & social care | 2020 |
| Joesven Imaginings Limited | Professional & scientific | 2025 |
| John Estate Limited | Other services | 2016 |
| Johnson Jasper Limited | Real estate | 2024 |
| Jondee Consulting Limited | Professional & scientific | 2005 |
| Jondee UK Investments Limited | Financial services | 2012 |
| Jones Fire Ltd | Construction | 2023 |
| Joon & Co Catering Limited | Hospitality | 2018 |
| Joon Food Limited | Hospitality | 2020 |
| Jordan UK Electricals Ltd | Construction | 2026 |
| Josh Jones Ltd | Professional & scientific | 2025 |
| JP Childcare Ltd | Health & social care | 2024 |
| JP Industries Ltd | Arts & recreation | 2017 |
| JP Investment Property Ltd | Real estate | 2022 |
| JPS Cole Limited | Professional & scientific | 2022 |
| JRY Ltd | Other services | 2022 |
| JSD Specialists Limited | Administrative & support | 2017 |
| JTG Civils Concrete Reinforcement Ltd | Construction | 2025 |
| JTM Holdings INT Limited | Financial services | 2019 |
| Jtxp Properties Ltd | Real estate | 2025 |
| Jumpstart I.T. Solutions Ltd | Information & communication | 2011 |
| Junior Cwac Cwac's Ltd | Education | 2025 |
| Just Covers South Wales Limited | Manufacturing | 2014 |
| Just Houses (SW) Limited | Real estate | 2017 |
| Justproveit Limited | Information & communication | 2025 |
| Justwin Marketing Ltd | Information & communication | 2018 |
| Jwlar Entertainment Ltd | Information & communication | 2020 |
| JWT Property Ltd | Real estate | 2019 |
| K & DC Properties Ltd | Real estate | 2023 |
| K H Barber Ltd | Other services | 2024 |
| K Pole Building Services Ltd | Construction | 2013 |
| K Surveyors Limited | Professional & scientific | 2024 |
| K'S Fresh Produce Limited | Wholesale & retail | 2009 |
| K. Gurner Builder Limited | Wholesale & retail | 1989 |
| K.G.J. Price (Railway Contractors) Limited | Manufacturing | 1983 |
| K.G.J. Price Holdings Limited | Financial services | 2023 |
| K8 Aesthetics Limited | Health & social care | 2022 |
| Kabin Hire Holdings Limited | Financial services | 2025 |
| Kabin Hire Limited | Construction | 1974 |
| Kala Brand Music UK Limited | Manufacturing | 2024 |
| Kardar Co Ltd | Hospitality | 2019 |
| Karls Access Stairlifts Limited | Wholesale & retail | 2017 |
| Karmon Nutrition Ltd | Manufacturing | 2025 |
| Karunadipathi Global Limited | Wholesale & retail | 2023 |
| Karve Design Ltd | Manufacturing | 2025 |
| Kate Stretch Consultancy Ltd | Other services | 2025 |
| Kaya Barbers Caerphilly Ltd | Other services | 2024 |
| KDF Home Improvements Limited | Construction | 2015 |
| Kearns Legal Services Ltd | Professional & scientific | 2013 |
| Kedward Consultancy Ltd | Hospitality | 2018 |
| KGM Construction Ltd | Real estate | 2025 |
| Khehra Store Ltd | Wholesale & retail | 2015 |
| Kieran Jones Solutions Ltd | Construction | 2024 |
| Kilner Morgan Ltd | Information & communication | 2018 |