| Mericon AS | Professional & scientific | 1990 |
| Mester Service AS | Real estate | 2018 |
| Mesto Byggservice AS | Construction | 2017 |
| Mesto Entreprenør AS | Construction | 2019 |
| Mesto Rødmyrlia AS | Real estate | 2019 |
| Mesto Utvikling AS | Real estate | 2017 |
| Metron Takst & Energi AS | Professional & scientific | 2026 |
| Mette Heines AS | Professional & scientific | 2022 |
| Mgø Holding AS | | 2018 |
| Mgø Invest AS | | 2020 |
| MH Ship Invest AS | | 2022 |
| Mh-Aktiva AS | Construction | 2021 |
| Mho Holding AS | | 2015 |
| Mi Aiuta AS | Education | 2024 |
| Micro Kompaniet AS | Wholesale & retail | 2003 |
| Midgaard Eiendom AS | Manufacturing | 1997 |
| Midt-Telemark Eigedom Og Utvikling AS | Real estate | 2020 |
| Miho AS | Construction | 2022 |
| Miljøbriketter AS | Manufacturing | 2021 |
| Miljøprosjekt AS | Professional & scientific | 2008 |
| Millba AS | Manufacturing | 1999 |
| Mime Protector AS | | 1998 |
| Mimre AS | Information & communication | 2025 |
| Minnesjord Holding AS | Real estate | 2008 |
| Mirajor Holding AS | | 2026 |
| Misso AS | Wholesale & retail | 2022 |
| Mitchell Holding AS | Construction | 2005 |
| Mithi Med AS | Health & social care | 2026 |
| Mitt & Ditt Velvære AS | Other services | 2014 |
| Mjcon AS | Professional & scientific | 2023 |
| Mjølner Bolig AS | Construction | 2021 |
| Mjølten Holding AS | | 2026 |
| Mjølten Invest AS | Real estate | 2026 |
| MK Property Management AS | Real estate | 2025 |
| MLT Holding AS | Real estate | 2013 |
| MM4 Consulting AS | Information & communication | 2021 |
| MM4 Management AS | Professional & scientific | 2024 |
| MMC Invest AS | Real estate | 2019 |
| Mobile Billakkering AS | Other services | 2012 |
| Mobile Skien AS | Other services | 1991 |
| Modesty Norway Da | Arts & recreation | 2008 |
| Modo Holding AS | | 2022 |
| Modulservice AS | Construction | 2012 |
| Modus Operandi Consulting AS | Professional & scientific | 2025 |
| Moen & Halvorsen Ans | Wholesale & retail | 2024 |
| Moflaten Smie Og Verksted AS | Manufacturing | 1969 |
| Moflatvegen 59 AS | Real estate | 2017 |
| Moltemyr Invest AS | | 2020 |
| Momentum Consult AS | Information & communication | 2020 |
| Momentum Invest AS | Real estate | 2000 |
| Momiji Sushi Skien AS | Hospitality | 2023 |
| Monami AS | | 2023 |
| Montesino Holding AS | Professional & scientific | 2024 |
| Moonbase AS | Information & communication | 2023 |
| Mor Og Barn Fysioterapi AS | Health & social care | 2014 |
| Moratel Elektro AS | Construction | 1998 |
| Morena AS | Wholesale & retail | 2016 |
| Morten Fjelldalen AS | Administrative & support | 2021 |
| Morten Goberg Lie Holding AS | Real estate | 2015 |
| Morten Grohs Holding AS | Real estate | 2016 |
| Morten Haugen Holding AS | Arts & recreation | 2014 |
| Morten Hegna Invest AS | Information & communication | 2005 |
| Morten Jensen Holding AS | | 2022 |
| Mosebø Eiendom AS | Real estate | 2020 |
| Mosebø Invest AS | Real estate | 2014 |
| Moss Bil & Båtpleie AS | Other services | 2024 |
| Moss Dagligvarer AS | Wholesale & retail | 2023 |
| Motexa AS | Wholesale & retail | 2026 |
| Motivasjonshuset Foredrag AS | Arts & recreation | 2024 |
| Motor & Gear AS | Wholesale & retail | 1984 |
| Motor Forum AS | Wholesale & retail | 1990 |
| Motor Huset AS | Wholesale & retail | 1985 |
| Motor Og Marineservice AS | Wholesale & retail | 2018 |
| Motor Technik AS | Other services | 2020 |
| Motor Technik Eiendom AS | Real estate | 2010 |
| Movella Da | Wholesale & retail | 2024 |
| Movementpass AS | Information & communication | 2020 |
| Moynora Holding AS | Real estate | 2021 |
| MPC Eiendom AS | Real estate | 2024 |
| MPC Eiendom Dalen AS | Real estate | 2026 |
| MPC Holding AS | Education | 2024 |
| MR. Phone AS | Other services | 2026 |
| MR. Wok Skien AS | Hospitality | 2008 |
| MRW Holding AS | Financial services | 2001 |
| MS Autoshine AS | Other services | 2023 |
| MS Bil AS | Wholesale & retail | 2025 |
| MSG Aerotech AS | Professional & scientific | 2024 |
| Mulighetenes by AS | Real estate | 2017 |
| Multiarena AS | Administrative & support | 2009 |
| Multiarena Sport Group AS | Wholesale & retail | 2013 |
| Multiassistanse AS | Other services | 2023 |
| Mumsy Investments AS | Hospitality | 2024 |
| Munsky Holding AS | | 2026 |
| Muon Avdeling Skien AS | Administrative & support | 2025 |
| Muotka&Schnabel AS | Information & communication | 2020 |
| Murer Lasse Johnsen AS | Construction | 2012 |
| Murerfirma Terje Norheim AS | Construction | 2020 |
| Murmester Hansen & Henriksen AS | Construction | 2020 |
| Murmester Kristian Karlsen AS | Construction | 2015 |
| Murmester Rune Ytre-Hauge AS | Construction | 2013 |