| Monmouth Payroll Services Limited | Professional & scientific | 2025 |
| Monmouth Rugby Football Club Limited | Arts & recreation | 2026 |
| Monmouth Self Store (Manson) Limited | Transport & logistics | 2016 |
| Monmouth Self Store Limited | Administrative & support | 2014 |
| Monmouth Sports Association Ltd | Arts & recreation | 2025 |
| Monmouth Therapy Limited | Health & social care | 2026 |
| Monmouthshire Country Cottages Limited | Construction | 2012 |
| Monmouthshire Hearing Centre Limited | Health & social care | 2022 |
| Monnow Bridge Caravan Site Ltd | Hospitality | 2024 |
| Monnow Construction and Renovation Ltd | Construction | 2026 |
| Monnow Consulting Limited | Professional & scientific | 2012 |
| Monnow Corporate Consulting Limited | Financial services | 2023 |
| Monnow Eco Plumb Ltd | Construction | 2023 |
| Monnow Ltd | Hospitality | 2018 |
| Monnow Marquees Limited | Construction | 2019 |
| Monnow Masonry Ltd | Construction | 2025 |
| Monnow Mowers & Machinery Limited | Wholesale & retail | 1994 |
| Monnow Property Lettings Ltd | Real estate | 2025 |
| Monnow Site Services Ltd | Construction | 2021 |
| Monnow Street Residents Management Company Limited | Households | 2019 |
| Monnow Valley Services Limited | Arts & recreation | 2006 |
| Monnow Voice Limited | Information & communication | 2009 |
| Montra Group Ltd | Wholesale & retail | 2025 |
| Moorcastle Limited | Real estate | 2013 |
| Morfa Management Limited | Professional & scientific | 1998 |
| Morgan Accountancy Limited | Professional & scientific | 2018 |
| Morgan Escott & Company Limited | Financial services | 2020 |
| Morris & Co. Real Estate and Lettings Agengy Ltd | Real estate | 2023 |
| Mortgage Taylors Limited | Financial services | 2003 |
| Morton Motors Limited | Wholesale & retail | 2013 |
| Mould Economics Limited | Professional & scientific | 2017 |
| Movente Limited | Real estate | 2000 |
| Mrjt Consultants Ltd | Administrative & support | 2020 |
| Mscoserv Ltd | Administrative & support | 2020 |
| Msct Trustee Limited | Other services | 2018 |
| My Fair Baby Boutique Ltd | Wholesale & retail | 2020 |
| Mylahmoo Ltd | Professional & scientific | 2018 |
| MYR Limited | Agriculture & fishing | 2003 |
| Nakm Property Ltd | Real estate | 2017 |
| Native Woodland Holdings Limited | Financial services | 2025 |
| Naughty Eats Ltd | Wholesale & retail | 2023 |
| Nero Engineering Wales Ltd | Professional & scientific | 2024 |
| Nerys Tudor Jones Primary D&T Consultancy Ltd | Education | 2022 |
| New Brooklyn Motors Limited | Wholesale & retail | 2004 |
| New York Street Food Ltd | Hospitality | 2016 |
| Newash Estates Ltd | Real estate | 2024 |
| Newways Ecology Ltd | Professional & scientific | 2024 |
| Next Bodies Limited | Other services | 2017 |
| Nicholls Consultancy Limited | Professional & scientific | 2024 |
| Nightingale Electrical Limited | Construction | 2021 |
| Nimbus Records Limited | Professional & scientific | 2006 |
| NJN Vehicle Solutions Limited | Wholesale & retail | 2013 |
| Noble Stone Properties Limited | Real estate | 2026 |
| Norfolk Garrett Consultancy Limited | Professional & scientific | 2013 |
| Novalux CHP Limited | Construction | 2023 |
| Novalux Energy North Ltd | Manufacturing | 2014 |
| Novalux Solar Limited | Construction | 2023 |
| Novira Heating Solutions Ltd | Construction | 2026 |
| NP25 Construction Limited | Construction | 2020 |
| NP25 Ltd | Information & communication | 2017 |
| NP253 Ltd | Hospitality | 2023 |
| NPT Property Ltd | Real estate | 2025 |
| Oakdene Residents Association (Monmouth) Limited | Households | 1985 |
| Oakhall Group Ltd | Construction | 2020 |
| Oakley Court Rogerstone Management Company Ltd | Real estate | 2023 |
| Oasis Software Solutions Limited | Information & communication | 2005 |
| Observator Ltd | Wholesale & retail | 2015 |
| OC Media Ltd | Information & communication | 2024 |
| ODG Comms Limited | Information & communication | 2025 |
| OFF Grid Heating Limited | Construction | 2014 |
| Ohar Ltd | Administrative & support | 2024 |
| OLB Creative Arts Limited | Arts & recreation | 2025 |
| OLB Property Solutions Limited | Real estate | 2026 |
| Old Coach House Distillery Limited | Manufacturing | 2019 |
| Oldunn Developments Ltd | Other services | 2017 |
| Oliveland Technology Ltd | Information & communication | 2023 |
| Omniorigin Limited | Construction | 2025 |
| One Stop Holiday Shop Limited | Real estate | 2022 |
| One Stop Poster Shop Ltd | Manufacturing | 2021 |
| One World Orchestra CIC | Arts & recreation | 2024 |
| ONE2ONE Nutrition Limited | Health & social care | 2005 |
| OPEN2MUSIC Limited | Administrative & support | 2010 |
| Orange Collar Media Ltd | Professional & scientific | 2025 |
| Order of Jacobs Well Ltd | Other services | 2016 |
| Oren CDW Ltd | Manufacturing | 2017 |
| Oskia Limited | Manufacturing | 2022 |
| Oskia Skincare Limited | Wholesale & retail | 2009 |
| OSS Software Limited | Information & communication | 2011 |
| OUT of Bounds Golf Ltd | Wholesale & retail | 2014 |
| Outlore Limited | Information & communication | 1995 |
| Owlka Ltd | Information & communication | 2026 |
| Own Your Change Ltd | Other services | 2025 |
| Oxford Coffee Company (Monmouth) Limited | Hospitality | 2018 |
| P & R Management Limited | Hospitality | 1993 |
| P J Bowen Motors Limited | Wholesale & retail | 2015 |
| P R Miles Consultants Ltd | Professional & scientific | 2016 |
| P. Badman & Co Limited | Wholesale & retail | 2002 |
| Panoramic Business Consultants Ltd | Professional & scientific | 2020 |
| Paper Mill Media Limited | Arts & recreation | 2014 |
| Parallel Accounting Ltd | Professional & scientific | 2019 |