| Starburst Stories | Professional & scientific | 1998 |
| Stasjonsdrift Blom AS | Wholesale & retail | 2019 |
| Stasjonsnett AS | Wholesale & retail | 2010 |
| State AS | Professional & scientific | 2019 |
| Statera Accounting AS | Professional & scientific | 2017 |
| Staut Entreprenør AS | Construction | 2018 |
| Stay Hotel Management AS | Arts & recreation | 2024 |
| Stea Norge AS | Wholesale & retail | 2015 |
| Steady AS | Professional & scientific | 2012 |
| Steen Gruppen AS | Construction | 2025 |
| Steinsprang Da | Information & communication | 2010 |
| Stema AS | Wholesale & retail | 1975 |
| Stema Bolig AS | Real estate | 1995 |
| Stema Bygg AS | Construction | 2016 |
| Sterkere Trening AS | Arts & recreation | 2020 |
| Stetinn AS | Real estate | 2010 |
| Stian Nævestad Holding AS | | 2024 |
| Stifab Norge AS | Education | 2010 |
| Stille AS | Wholesale & retail | 2023 |
| Stillverket Tomteselskap AS | Real estate | 2025 |
| Stillverket Utbygging AS | Real estate | 2025 |
| Stillverksveien 18/26 AS | Real estate | 2021 |
| Storgata 13A AS | Real estate | 2020 |
| Storgata Holding AS | Real estate | 2024 |
| Storgata Legesenter Lillestrøm AS | Health & social care | 2020 |
| Storgata Optikk AS | Wholesale & retail | 2006 |
| Storm Drift AS | Real estate | 2000 |
| Stortorget 10 AS | Real estate | 2025 |
| Storvik & Sivertsen Ans | Wholesale & retail | 1995 |
| Stralar Eiendom AS | Real estate | 2013 |
| Strawberry Invest AS | Financial services | 2021 |
| Strømmen Bygg Jobs AS | Construction | 2023 |
| Strømmen Digitale Løsninger AS | Information & communication | 2021 |
| Strømmen Entreprenør AS | Real estate | 1961 |
| Strømsveien 66 B AS | Real estate | 2011 |
| Strømsveien 76 AS | Real estate | 2016 |
| Studio Alf Lillestrøm AS | Other services | 1986 |
| Studio Estetikk AS | Other services | 2025 |
| Studio Oscar AS | Arts & recreation | 1995 |
| Studio Ren AS | Professional & scientific | 2025 |
| Studio Saasen AS | Information & communication | 2015 |
| Sucus AS | Construction | 2007 |
| Sullycon AS | Wholesale & retail | 2022 |
| Sunn Tannhelseklinikk AS | Health & social care | 2016 |
| Sunnysideup AS | | 2024 |
| Super Geni AS | Real estate | 2022 |
| Supero Performance AS | Wholesale & retail | 2022 |
| Supiter Holding AS | Information & communication | 2021 |
| Sushi Asia Giang AS | Hospitality | 2011 |
| Susi Invest AS | | 2021 |
| Suua Eiendom AS | Real estate | 2022 |
| Suua Holding AS | Real estate | 2020 |
| Sveis Og Rep AS | Manufacturing | 1984 |
| Sven Aas Elektro-Diesel AS | | 1980 |
| Svensen Maskin AS | Construction | 1965 |
| Sverres Gate 7B AS | Real estate | 2023 |
| Swairi AS | Information & communication | 2016 |
| Swecon AS | Construction | 1996 |
| Swemac Orthopaedics AS | Wholesale & retail | 2004 |
| Swing IT Holding AS | Financial services | 2015 |
| Syd AS | Real estate | 2007 |
| Syklus Invest AS | Financial services | 2016 |
| Symphony Gems AS | Wholesale & retail | 2023 |
| Synhaug Eiendom AS | Professional & scientific | 2021 |
| Sølepytten Barnehage AS | Education | 2017 |
| Søndre Utgård Va Utbygging SA | Water & waste | 2018 |
| Sørum VVS AS | Construction | 2017 |
| Sørumsgata 11A AS | Real estate | 2013 |
| Sørumsgata 33 AS | Real estate | 2021 |
| Sørumsgata 76 AS | Real estate | 2022 |
| Sørumsparken Invest AS | Administrative & support | 2020 |
| Søstrene Gill AS | Wholesale & retail | 2008 |
| Søylen BSK AS | Professional & scientific | 2021 |
| T.K Eiendom & Rør AS | Real estate | 2013 |
| T2 Eiendom AS | Real estate | 2024 |
| T3 Næringseiendom AS | Real estate | 2024 |
| T4 Eiendom AS | Real estate | 2024 |
| Tabellae AS | Information & communication | 2016 |
| Taksteksperten AS | Real estate | 2014 |
| Takørn AS | Administrative & support | 2026 |
| Talien Invest AS | Financial services | 2017 |
| Tank Eiendom AS | Real estate | 2018 |
| Tank Holding AS | Transport & logistics | 2018 |
| Tankcontainertransport AS | Transport & logistics | 1998 |
| Tannlege Jan Boiesen AS | Health & social care | 2015 |
| Tannlege Khazaie AS | Health & social care | 2019 |
| Tannlege Pia Langdalen AS | Health & social care | 2014 |
| Tannlege S. Fallah AS | Health & social care | 2013 |
| Tannlegehuset Lillestrøm AS | Health & social care | 2025 |
| Tannlegene Depotgata 20 AS | Health & social care | 2024 |
| Tannlegene I Concordiabygget AS | Health & social care | 2006 |
| Tapas I Lillestrøm AS | Hospitality | 2025 |
| Taraldset Eiendom AS | Real estate | 1996 |
| Taraldset Holding AS | Construction | 2005 |
| Tasso AS | Information & communication | 2026 |
| Taste of Class Agency Da | Arts & recreation | 1991 |
| Taste of Ice Cream AS | Hospitality | 2012 |
| Tavex Gull Og Sølv AS | Wholesale & retail | 2014 |
| Tavlebyggern AS | Manufacturing | 2010 |
| TB Holding AS | Real estate | 2008 |