| Meraki Bespoke Hair Ltd | Other services | 2023 |
| Meraki Property Group Ltd | Real estate | 2019 |
| Mercia Publishing Ltd | Information & communication | 2023 |
| Merctech Limited | Wholesale & retail | 2003 |
| Mercury Carpets & Flooring Ltd | Wholesale & retail | 2016 |
| Meredith & Co UK Ltd | Real estate | 2026 |
| Merge AB UK Ltd | Manufacturing | 2025 |
| Meridian Coalhouse Ltd | Construction | 2022 |
| Meridian Health Partners Ltd | Health & social care | 2026 |
| Meridian Quay Ltd | Real estate | 2018 |
| Meridian Service Solutions Ltd | Hospitality | 2025 |
| Meridias Technologies Limited | Information & communication | 2026 |
| Merit Company Holdings Limited | Other services | 2025 |
| Merlin Fire Holdings Limited | Financial services | 2020 |
| Merlin Fire Services Limited | Construction | 2016 |
| MES Enfants Limited | Education | 2002 |
| Meshx Software Ltd | Wholesale & retail | 2023 |
| Mesopotamia Services Limited | Extraterritorial | 2018 |
| Mesq Painting Limited | Construction | 2025 |
| Messrs Davies & Evans Limited | Wholesale & retail | 2010 |
| Meta Software Ltd. | Information & communication | 2022 |
| Metal Masters (West Wales) Ltd | Manufacturing | 2007 |
| Metiq Ltd | Manufacturing | 2025 |
| Metlac Ltd | Real estate | 2020 |
| Metro Homes Lettings Ltd | Real estate | 2016 |
| Metropole Property Investments Ltd | Real estate | 2008 |
| Mettoy Company Plc(The) | | 1932 |
| Metz Home & Building Improvements Ltd | Construction | 2023 |
| MEW Homes Ltd | Real estate | 2021 |
| MEW Properties Ltd | Real estate | 2019 |
| MFA Brands Limited | Wholesale & retail | 2024 |
| MFO Design Ltd | Construction | 2020 |
| MG Beauty Studio Ltd | Other services | 2024 |
| MG Installations (Swansea) Limited | Other services | 2024 |
| MGH Consultancy Services Limited | Professional & scientific | 2019 |
| MGJ Brickwork Limited | Construction | 2026 |
| MGJ Carpentry Ltd | Manufacturing | 2025 |
| MGM Entertainment Limited | Arts & recreation | 2024 |
| MGN Lettings Ltd | Real estate | 2026 |
| MGR Motors Limited | Wholesale & retail | 2007 |
| MGS Engineering (South Wales) Limited | Construction | 2014 |
| MH 998 Limited | Hospitality | 2026 |
| Mh-Technip Limited | Professional & scientific | 2020 |
| MHA Independent Ltd | Health & social care | 2026 |
| MHG Swansea Limited | Construction | 2025 |
| MHM Plant Group Limited | Wholesale & retail | 2010 |
| MHT Construction Limited | Construction | 2023 |
| MI Prints Ltd | Wholesale & retail | 2024 |
| MI Total Physio Ltd | Health & social care | 2024 |
| Miah's Carpentry Services Limited | Construction | 2020 |
| Michael Austin EYE Care Limited | Health & social care | 2019 |
| Michael Baker Boiler Maintenance Limited | Construction | 2005 |
| Michael Cann Investments Limited | Households | 2025 |
| Michael Developments Limited | Real estate | 2025 |
| Michael J Thomas Limited | Administrative & support | 2009 |
| Michael Noir Hair Ltd | Other services | 2025 |
| Michael Sosner Ltd | Hospitality | 2018 |
| Michael Williams Substation Access Limited | Manufacturing | 2015 |
| Michaela Breeze Ltd | Education | 2022 |
| Micro Computer Associates Limited | Information & communication | 1985 |
| Microwave Testing Limited | Professional & scientific | 2025 |
| Midas Green Innovations Ltd | Professional & scientific | 2019 |
| Midas Marketing Limited | Information & communication | 2018 |
| Middlemass Consultancy Services Limited | Professional & scientific | 2015 |
| Middleton Real Estate Limited | Real estate | 2017 |
| Midgard Engineering and Design Ltd | Manufacturing | 2022 |
| Midgard IT Limited | Other services | 2011 |
| Midland Improvements Ltd | Construction | 2019 |
| Midpoint Engineering Limited | Manufacturing | 2026 |
| Midtown Store Ltd | Wholesale & retail | 2024 |
| Midtown Vape Shop Ltd | Wholesale & retail | 2026 |
| Miers Mechanical Fitter Limited | Construction | 2024 |
| Migael Antony Ltd | Health & social care | 2025 |
| Mighty Atom Productions Ltd. | Information & communication | 1999 |
| Mike and SON Auto Mechanics Ltd | Wholesale & retail | 2026 |
| Mike Barnes Engineering Ltd | Professional & scientific | 2015 |
| Mike Cooke Limited | Wholesale & retail | 2018 |
| Mike Dempster Inspection Services Ltd | Professional & scientific | 2019 |
| Mike Grey Financial Services Limited | Financial services | 2015 |
| Mike Tucker Autos and SON Ltd | Wholesale & retail | 2021 |
| Mike Woods Consulting Limited | Professional & scientific | 2012 |
| Mikes on the Move Removals Limited | Transport & logistics | 2017 |
| Mikoshi Ltd | Other services | 2022 |
| Milano's Swansea Ltd | Hospitality | 2025 |
| Milanos Morriston Ltd | Hospitality | 2024 |
| Miles Better Homes Limited | Real estate | 2020 |
| Miles Court Residents Limited | Real estate | 2002 |
| Miles Hire Limited | Administrative & support | 2003 |
| Milestone Property Investments Ltd | Real estate | 2025 |
| Milford Flats Limited | Real estate | 2016 |
| Milford Steel Stockholders Limited | Extraterritorial | 2002 |
| Milhg CYF | Professional & scientific | 2021 |
| Mill Gardens Estates Limited | Hospitality | 2013 |
| Mill House Caravan & Camping Limited | Hospitality | 2023 |
| Millberry Associates Limited | Construction | 2023 |
| Millennium Capital Ventures Ltd | Professional & scientific | 2009 |
| Millennium Lasers Ltd | Administrative & support | 2015 |
| Miller & Partner Limited | Financial services | 2025 |
| Millstones Estates Ltd | Construction | 2026 |
| Milmai Projects Ltd | Professional & scientific | 2025 |