| China Twang AS | Hospitality | 2007 |
| Choose AS | Real estate | 2006 |
| CHR H Blomsgate AS | Real estate | 2013 |
| Chris Frisør AS | Other services | 2017 |
| Christian Thorsbye AS | Professional & scientific | 2012 |
| Christiansen & Christiansen Bygg AS | Construction | 2012 |
| Christiansen Transport AS | Transport & logistics | 2023 |
| Chrito AS | Real estate | 2005 |
| Chromo AS | Information & communication | 2026 |
| Chuan Chim Thai Food AS | Hospitality | 2022 |
| Cial AS | Wholesale & retail | 2009 |
| Cingo Holding AS | | 2025 |
| Cir.Cuz Ent. AS | Arts & recreation | 2017 |
| Citadel AS | Real estate | 2018 |
| Citadel Skien AS | Real estate | 2026 |
| City Asiamat AS | Wholesale & retail | 2020 |
| City Bil Skien AS | Wholesale & retail | 2016 |
| City Grill AS | Hospitality | 2023 |
| Citybygg AS | Real estate | 1970 |
| CJB Forvaltning AS | Professional & scientific | 2024 |
| CJL Holding AS | | 2008 |
| CJL Holding II AS | | 2026 |
| Claj Holding AS | Wholesale & retail | 2018 |
| Claussen Eiendom AS | Real estate | 2004 |
| Claussen Elektro AS | Construction | 2021 |
| Claussen Holding AS | Construction | 2018 |
| Clearnord AS | Information & communication | 2025 |
| CM Solutions AS | Professional & scientific | 2021 |
| Coanor Consulting AS | Information & communication | 2024 |
| Cobber Holding AS | | 2025 |
| Coco Skien AS | Wholesale & retail | 2000 |
| Codx Holding AS | | 2012 |
| Cody AS | Manufacturing | 2011 |
| Colliflow AS | Transport & logistics | 2021 |
| Columbi Regnskap AS | Professional & scientific | 1988 |
| Com Con AS | Hospitality | 1996 |
| Constella AS | | 2025 |
| Constella Terra AS | | 2025 |
| Container 1 AS | Administrative & support | 2023 |
| Contex AS | Real estate | 2026 |
| Convo Utvikling AS | Information & communication | 2024 |
| Conzes AS | Real estate | 2019 |
| Coprian AS | Real estate | 2025 |
| Core Company Da | Wholesale & retail | 2026 |
| Cort Adelersgate 6 AS | Real estate | 2015 |
| Cotton Child AS | Wholesale & retail | 1997 |
| Cowpower AS | Real estate | 2013 |
| Cpwammer Invest AS | | 2026 |
| CR Group AS | Professional & scientific | 2011 |
| Crepella AS | Hospitality | 2021 |
| Crestano AS | Wholesale & retail | 2005 |
| CRG Invest AS | Construction | 2014 |
| Crime Records AS | Information & communication | 2013 |
| Crime Wine AS | Wholesale & retail | 2020 |
| Crumena AS | Real estate | 2010 |
| Crystallize AS | Information & communication | 2017 |
| CS Graving Og Transport AS | Construction | 2018 |
| CSD Invest AS | Wholesale & retail | 2022 |
| Csi, Christian Shriwer International AS | Professional & scientific | 1995 |
| Cura Holding AS | Real estate | 2015 |
| Cura Kompetanse AS | Health & social care | 2015 |
| Cura Omsorg AS | Health & social care | 2023 |
| Cyber Wolf AS | Information & communication | 2021 |
| Cycled Technologies AS | Information & communication | 2016 |
| Cylinder Tech AS | Professional & scientific | 2015 |
| Cypro AS | Professional & scientific | 2020 |
| D Borgersen Holding AS | Wholesale & retail | 2025 |
| Daci Invest AS | | 2026 |
| Dacom Aircon AS | Wholesale & retail | 2011 |
| Dag Olav Brekke AS | Real estate | 1991 |
| Dagh Consulting AS | Professional & scientific | 2016 |
| Dagligdata AS | Information & communication | 2003 |
| Dale Anlegg AS | Construction | 2014 |
| Dalen Import Eva Og Lars Dalen Ans | Wholesale & retail | 1987 |
| Dalenveien 9 AS | Real estate | 2026 |
| Damfoss AS | Real estate | 2001 |
| Dampern AS | Wholesale & retail | 2020 |
| Danichri Invest AS | Financial services | 2014 |
| Daniel's Tak Og Byggservice AS | Construction | 2014 |
| Dansken AS | Construction | 2023 |
| Dantor Holding AS | Construction | 2014 |
| Daro Eiendom Ans | Real estate | 2011 |
| Dattebayo Gruppen AS | Real estate | 2017 |
| Dattebayo Skien AS | Hospitality | 2017 |
| Davinsji Kapital AS | | 2019 |
| DB Bygg & Montasje AS | Construction | 2017 |
| Default Company AS | Information & communication | 2021 |
| Dekalshoppen AS | Professional & scientific | 2024 |
| Dekkspesialisten Skien AS | Wholesale & retail | 2002 |
| Delfin SK AS | Education | 2017 |
| Den Lille Sorte AS | Wholesale & retail | 2005 |
| Den Nye Trafikkskolen AS | Education | 2016 |
| Designa Skien AS | Wholesale & retail | 2017 |
| Detalj Konsulentene Eiendom AS | Real estate | 2001 |
| Devlon AS | Wholesale & retail | 2024 |
| Dewl Holding AS | | 2022 |
| DG Byggrenovering AS | Construction | 2019 |
| DG Konsulenten AS | Professional & scientific | 2015 |
| Di Consulting AS | Professional & scientific | 2010 |
| Di Napoli AS | Hospitality | 2024 |