| J Reynolds Design & Build Limited | Construction | 2021 |
| J&G International Food Ltd | Wholesale & retail | 2024 |
| J.M. Plastics Limited | Manufacturing | 1988 |
| James & James Properties Ltd | Real estate | 2022 |
| James Domestic GAS Services Ltd | Construction | 2021 |
| James Integrated Integrity Services Ltd | Manufacturing | 2026 |
| Jason LEE Personal Training Limited | Education | 2026 |
| Jeffs Jewellers Ltd | Wholesale & retail | 2022 |
| JER Consulting Limited | Information & communication | 2012 |
| Jesse Button Carpentry Ltd | Construction | 2026 |
| Jgrj Interiors Ltd | Manufacturing | 2026 |
| JJS Property Management Limited | Real estate | 2025 |
| JK Construction & Carpentry Limited | Construction | 2024 |
| JKS Construction Roads and Rails Ltd | Construction | 2017 |
| JNB Autos Limited | Wholesale & retail | 2025 |
| Jo's Newsagents Limited | Wholesale & retail | 2008 |
| Jones & Phillips Property Maintenance Ltd | Real estate | 2019 |
| Jones Courier Solutions Ltd | Transport & logistics | 2025 |
| Jonez Properties Ltd | Real estate | 2023 |
| JP Drywall Ltd | Construction | 2015 |
| JS and DS Developments Ltd | Construction | 2025 |
| Jukebox Covers Band UK Ltd | Arts & recreation | 2013 |
| Junior Cameron Rail Ltd | Construction | 2016 |
| K Gold Couriers Ltd | Transport & logistics | 2025 |
| K Thomas Ltd | Health & social care | 2016 |
| K4 Property Group Ltd | Real estate | 2023 |
| Kanty Security Service Ltd | Administrative & support | 2019 |
| Karl Langmead Surfacing Ltd | Construction | 2023 |
| Karoly Solutions Limited | Administrative & support | 2014 |
| Katie Derrick Aesthetics Ltd | Other services | 2025 |
| Katrick Capital Investments Ltd | Real estate | 2024 |
| Key Brothers Property Developers Limited | Real estate | 2025 |
| Key Roofing Contractors Ltd | Construction | 2020 |
| Key Tanning Ltd | Other services | 2023 |
| Kfworks Ltd | Energy | 2025 |
| Kingsbridge Residential Lettings Ltd | Real estate | 2026 |
| Kirstys Kandy Limited | Hospitality | 2015 |
| KJ Retail Ltd | Wholesale & retail | 2019 |
| KL Highway Engineering Ltd | Construction | 2025 |
| KLA Stone & Kitchen Renovations Ltd | Construction | 2024 |
| Klhp Property Ltd | Wholesale & retail | 2024 |
| KML Burgers Limited | Hospitality | 2025 |
| Knowles Roofing Services Ltd | Construction | 2024 |
| KSG Networks Ltd. | Information & communication | 2010 |
| KT Logistics Limited | Transport & logistics | 2025 |
| L. E. Morris Ltd | Professional & scientific | 2016 |
| L.J. Design Solutions Ltd | Administrative & support | 2016 |
| Langmead Construction Limited | Construction | 2004 |
| Lavish Beauty by Liza Byrne Ltd | Other services | 2019 |
| LBR Groundworks Ltd | Construction | 2025 |
| Leeway Productions Limited | Arts & recreation | 2016 |
| Lewis & Phelan Properties Ltd | Real estate | 2024 |
| Lezano’S Barber Shop Ltd | Other services | 2022 |
| Libbrooke Developments Ltd | Real estate | 2025 |
| Liberty Camps Conversions Ltd | Manufacturing | 2025 |
| Little Taylor’S Baby Boutique Ltd | Wholesale & retail | 2022 |
| LJB Weld Ltd | Construction | 2022 |
| Llantrisant Self Storage Limited | Real estate | 2016 |
| Llanwonno Estates Limited | Real estate | 2019 |
| Llanwonno Holdings Limited | Financial services | 2025 |
| Llanwonno Residential Estates Limited | Real estate | 2025 |
| LMJ Digital Ltd | Manufacturing | 2019 |
| LMT Academy Ltd | Arts & recreation | 2026 |
| Local Wales Lettings Ltd | Real estate | 2019 |
| Look Goode Limited | Other services | 2019 |
| Louch Electrical Limited | Energy | 2017 |
| Lougar Developments Limited | Real estate | 2026 |
| Love Life Love YOU Limited | Other services | 2015 |
| M C Electrical Contractors Limited | Construction | 2003 |
| M&H 4X4 Ltd | Wholesale & retail | 2025 |
| M&H 4X4 Services Ltd | Wholesale & retail | 2025 |
| M.E Publications and Print Ltd | Professional & scientific | 2024 |
| M.N Rail Ltd | Construction | 2018 |
| MA Homes to LET Ltd | Real estate | 2025 |
| MA Ocular Services Ltd | Health & social care | 2025 |
| Maani Byte Ltd | Information & communication | 2026 |
| Mafrail Ltd | Wholesale & retail | 2023 |
| Makeupbyjessevans Ltd | Professional & scientific | 2023 |
| Management Consultants Worldwide Ltd | Administrative & support | 2021 |
| Marc Taylor Driver Services Ltd | Transport & logistics | 2020 |
| Marc Travels Ltd | Transport & logistics | 2025 |
| Mark J Consultancy Limited | Information & communication | 2010 |
| Martin-Price Consultants Ltd | Professional & scientific | 2023 |
| Matthew O'Keeffe Contracting Limited | Construction | 2017 |
| MC Doner Valley Ltd | Hospitality | 2020 |
| MCP49 Ltd | Hospitality | 2026 |
| ME Invest Ltd | Real estate | 2025 |
| Meadbro Construction Limited | Construction | 1979 |
| Mecpipe Installations Limited | Professional & scientific | 2015 |
| MGH Mechanical Services Ltd | Mining & quarrying | 2025 |
| Mhim Limited | Wholesale & retail | 2010 |
| Mike Bailey Heating Ltd | Construction | 2014 |
| Millsmoves Limited | Real estate | 2026 |
| MIM Electrical Services Ltd | Construction | 2023 |
| Minecope Limited | Agriculture & fishing | 1988 |
| Mini Mayhem Softplay Ltd | Administrative & support | 2024 |
| Mintech Construction Services Limited | Professional & scientific | 2009 |
| Missionz Ltd | Wholesale & retail | 2024 |
| MJB Servicing Ltd | Construction | 2020 |
| MJR Distributions Ltd | Wholesale & retail | 2026 |