| Torggata 7 Eiendom AS | Real estate | 2021 |
| Torggata 9 Bolig AS | Real estate | 2019 |
| Torggata 9 Skien AS | Real estate | 2017 |
| Torje Holding AS | | 2021 |
| Torjusrød Skogveglag SA | Real estate | 1963 |
| Torsvang Maskin AS | Other services | 1991 |
| Tos Invest AS | Real estate | 2021 |
| Total Elektro AS | Real estate | 1993 |
| Total Sparing AS | Professional & scientific | 2019 |
| Total System Avdeling Skien - Porsgrunn Ans | | 1988 |
| TP Holding AS | Construction | 2019 |
| TPK Invest AS | Financial services | 2005 |
| Trafikalt Simulator Senter AS | Education | 2025 |
| Tran Invest AS | | 2018 |
| Transport Og Renovasjon Da | Transport & logistics | 1994 |
| Transportprodukter AS | Wholesale & retail | 1997 |
| Transportsentralen Telemark AS | Transport & logistics | 2012 |
| Transportservice Skien AS | Transport & logistics | 1997 |
| Trapes Kiropraktorklinikk AS | Health & social care | 2019 |
| Trappegruppen Kapital AS | | 2025 |
| Trarin AS | Real estate | 2005 |
| Trattoria Lorenzo Ristirante AS | Hospitality | 2019 |
| Tre Løver AS | | 2020 |
| Trebua Invest AS | Information & communication | 1995 |
| Tredje AS | Professional & scientific | 2020 |
| Treff Invest AS | Real estate | 1992 |
| Treff Invest Eiendom AS | Real estate | 1994 |
| Trend Collection AS | Wholesale & retail | 2002 |
| Trend Sko Arkaden AS | Wholesale & retail | 1987 |
| Trend Sko AS | Wholesale & retail | 1993 |
| Trend Sko Brotorvet AS | Wholesale & retail | 2012 |
| Trend Sko Down Town AS | Wholesale & retail | 1988 |
| Trend Sko Larvik AS | Professional & scientific | 1994 |
| Trescon AS | Construction | 2017 |
| Trgi Holding AS | | 2025 |
| Triangelen Eiendomsselskap AS | Real estate | 1995 |
| Trigg AS | Professional & scientific | 2018 |
| Trikk AS | | 2020 |
| Trillepiken AS | Education | 2018 |
| Trim Grenland AS | Arts & recreation | 2011 |
| Trimco Group (Norway) AS | Professional & scientific | 2017 |
| Trio Montasje & Utleie AS | Real estate | 1974 |
| TRIPLE20 AS | Real estate | 2025 |
| Troens Liv Barnehage AS | Education | 2022 |
| Trollsåsen Skogsbilveglag SA | Real estate | 2026 |
| Trommedalsjordet 11 AS | Real estate | 2019 |
| Trommedalsjordet 2 AS | Real estate | 2021 |
| Trommedalsjordet AS | Real estate | 2018 |
| Trommedalsjordet Tomt 7 AS | Real estate | 2019 |
| Trommedalsjordet Tomt 9 AS | Real estate | 2019 |
| Trommedalsjordet Vest Eiendom AS | Manufacturing | 2016 |
| Trommedalskåsa 23 AS | Real estate | 2018 |
| Trommedalsveien 235 AS | Real estate | 2021 |
| Trommedalsveien Eiendom AS | Real estate | 2016 |
| Truck Teknikk Ans | Wholesale & retail | 2001 |
| Truls Og Trine Hårsenter Da | Other services | 1994 |
| Trumf Trading AS | Real estate | 1988 |
| TS Finans AS | Administrative & support | 2005 |
| TS Service & Event Da | Hospitality | 2019 |
| TS Sport Invest AS | Real estate | 2021 |
| Ts-Holding AS | Real estate | 2006 |
| TSB AS | Administrative & support | 2015 |
| TSF Invest AS | | 2026 |
| Tu Invest AS | Professional & scientific | 2011 |
| Tuhema Holding AS | | 2022 |
| Tunet Familiebarnehage Ans | Education | 1999 |
| TVB Bolig AS | Construction | 2004 |
| TVB Tomter AS | Real estate | 2013 |
| TVF Holding AS | | 2018 |
| Two Monocles AS | Wholesale & retail | 2014 |
| Tyri Skrivestove AS | Real estate | 2021 |
| Tømrer Magnar Larsen AS | Construction | 1987 |
| Tømrer Magne Nilsen AS | Construction | 2012 |
| Tømrer Stein Erik Andersen AS | Construction | 2024 |
| Tømrer William Christensen AS | Construction | 2021 |
| Tømreren Skien AS | Construction | 2012 |
| Tømrerfirma Fjellhøy AS | Construction | 2019 |
| Ubertas Holding AS | | 2021 |
| Ubri Medical AS | Wholesale & retail | 2016 |
| Ufs Holding AS | Professional & scientific | 2024 |
| Ugla Bokantikvariat Ans | Wholesale & retail | 2019 |
| Uglemor Holding AS | | 2025 |
| Ulefoss Number One AS | Construction | 2024 |
| Ulefossvegen 94 AS | Real estate | 2022 |
| Ulefossveien 42 AS | Real estate | 1946 |
| Ulev Bygg AS | Construction | 2023 |
| Ullevålslegene AS | Administrative & support | 2013 |
| Ulveseth Media AS | Professional & scientific | 2025 |
| Un Holding AS | Information & communication | 2005 |
| Under Pizza AS | Hospitality | 2024 |
| Unic Nettverk AS | Information & communication | 1992 |
| Union Bar AS | Hospitality | 2024 |
| Union Pallets AS | Manufacturing | 2008 |
| Union Plastic Industries AS | Manufacturing | 2006 |
| Unirep AS | Manufacturing | 2024 |
| Unirør AS | Construction | 2023 |
| Uniwater Holdco AS | Real estate | 2024 |
| Uniwise Norge | Information & communication | 2014 |
| Uphill Eiendom AS | Real estate | 2022 |
| Urmaker Skorve AS | Wholesale & retail | 1999 |