| Haminan Autokorjaamo Oy | Other services | 2011 |
| Haminan Energia Oy | Professional & scientific | 2024 |
| Haminan Erikoisrakennus Oy | Construction | 1998 |
| Haminan Freda Oy | Construction | 1993 |
| Haminan Fysioterapia Oy | Health & social care | 1980 |
| Haminan Hotellikiinteistöt Oy | Real estate | 2002 |
| Haminan Höyrylaiva Oy | Hospitality | 1981 |
| Haminan Isännöintipalvelu Oy | Real estate | 1987 |
| Haminan Kaukolämpö Oy | Energy | 2021 |
| Haminan Kiinteistö Oy | Real estate | 2003 |
| Haminan Kirppis ja Antiikki Oy | Wholesale & retail | 2024 |
| Haminan Luontaistuote Ky | Wholesale & retail | 1986 |
| Haminan Maansiirto Oy | Construction | 2000 |
| Haminan Merivesilaitos Oy | Water & waste | 2023 |
| Haminan Palloiluhalli Oy | Arts & recreation | 2014 |
| Haminan Parketti Ky | Construction | 1985 |
| Haminan Petkele OY | Real estate | 2023 |
| Haminan Portti Oy | Real estate | 1988 |
| Haminan Puupojat Oy | Wholesale & retail | 2008 |
| Haminan Rakentajapalvelu Ky | Construction | 1984 |
| Haminan Sementtituote Ky | Manufacturing | 1987 |
| Haminan Sunrise Oy | Hospitality | 2022 |
| Haminan Sähköverkko Oy | Energy | 2021 |
| Haminan Tehoradio Ky | Wholesale & retail | 2005 |
| Haminan Torigrilli Oy | Hospitality | 2023 |
| Haminan Troikka Yhtiöt Oy | Hospitality | 1996 |
| Haminan Vaunuhalli Oy | Real estate | 1987 |
| Haminan Vesijohtoliike Penttilä Oy | Construction | 2015 |
| Haminan Vuokrakiinteistöt Oy | Real estate | 2025 |
| Haminan Wanha Työski Oy | Hospitality | 2019 |
| Haminan mainos ja media Oy | Information & communication | 2022 |
| Hamjets Oy | Real estate | 1990 |
| Hammasfocus Oy | Health & social care | 2006 |
| Hammaslaboratorio J Åhman Ky | Manufacturing | 1996 |
| Hanita Oy | Transport & logistics | 2008 |
| Haveputki Oy | Construction | 1982 |
| HeMa Holding Oy | Professional & scientific | 2024 |
| Hejos Oy | Administrative & support | 1999 |
| Helmikuja Oy | Real estate | 2016 |
| Henver Oy | Administrative & support | 2017 |
| Here And Now Consulting Oy | Professional & scientific | 2019 |
| HevosHovi Oy | Arts & recreation | 2005 |
| Hevoshaan Teollisuuskiinteistö Oy | Real estate | 2020 |
| Hietanen Cargo Service Oy | Transport & logistics | 1994 |
| Hinausmestarit Oy | Professional & scientific | 2024 |
| HodLease Oy | Administrative & support | 2021 |
| Hotellioperaattorit Oy | Hospitality | 2002 |
| Hotmac Oy | Real estate | 1993 |
| Hovi-Invest Oy | Real estate | 2022 |
| Huolehtimo Oy | Health & social care | 2026 |
| Hyvänmielen Koirakeskus Oy | Other services | 2021 |
| Into Marketing Ky | Administrative & support | 2008 |
| IRCH Oy | Manufacturing | 2012 |
| Iblok Finland Oy | Construction | 2014 |
| Infraseppä Invest Oy | Financial services | 2025 |
| Insinööritoimisto Yrjö Saarelainen Oy | Professional & scientific | 1989 |
| Investuuri Oy | Financial services | 1988 |
| Itä-länsi trade Oy | Wholesale & retail | 2010 |
| Itäväylän Bensis Oy | Hospitality | 2007 |
| Itäväylän Jakeluasema P. ja T. Karhinen Ky | Wholesale & retail | 1992 |
| J & T Puutyöt Ky | Construction | 2018 |
| J&I Enterprise Oy | Education | 2024 |
| J. Purho Oy | Wholesale & retail | 1978 |
| JH Sukellus- ja Infrapalvelut Oy | Construction | 2025 |
| JH-Wise Engineering Oy | Professional & scientific | 2023 |
| JJ-LIT Oy | Professional & scientific | 2020 |
| JK In House Oy | Real estate | 2017 |
| JK LVI-Palvelut OY | Construction | 2021 |
| JKH Yhtiöt Oy | Real estate | 2019 |
| JLankila Oy | Construction | 2025 |
| JN Adviser Oy | Professional & scientific | 2024 |
| JN Liikenne Ky | Transport & logistics | 2017 |
| JP Style Oy | Manufacturing | 2012 |
| JP-Instituutti Oy | Professional & scientific | 1995 |
| JTT-Group Oy | Real estate | 1997 |
| JURSVEL Oy | Transport & logistics | 2025 |
| JUVE Suunnittelu Oy | Professional & scientific | 2019 |
| Jaakko Leislahti Oy | Information & communication | 2005 |
| Jaanan Ompelimo ja Asuste Ky | Manufacturing | 1990 |
| JaffeVet Oy | Professional & scientific | 2021 |
| Jang Stail M-L Hovi Ky | | 1986 |
| Jarkon Kala ky | Wholesale & retail | 2011 |
| Jarmitva avoin yhtiö | Wholesale & retail | 2000 |
| Jarmo Huotari Impex Oy | Professional & scientific | 1994 |
| Jave-Kaivuu Oy | Construction | 1999 |
| Jindustry Oy | Wholesale & retail | 2021 |
| Joonas Juhani & Partners Oy | Construction | 2021 |
| Joonas Juhani Holding Oy | Real estate | 2025 |
| Joosepin Ratkaisut Oy | Professional & scientific | 2026 |
| Jouko ja Pirjo-Riitta Ekman Oy | Real estate | 1979 |
| Jouni Hatonen kommandiittiyhtiö | Transport & logistics | 1997 |
| Jp tuonti oy | Administrative & support | 2024 |
| JuHu Oy | Transport & logistics | 2008 |
| K-Storm Oy | Transport & logistics | 2020 |
| K. Holopainen Kommandiittiyhtiö | Transport & logistics | 1977 |
| KISSELENKO Oy | Manufacturing | 2011 |
| KK Tuotanto Osk | Arts & recreation | 2019 |
| KLM Power Oy | Administrative & support | 2022 |
| KMT-Export Oy | Wholesale & retail | 2018 |
| KOY Haminan Pikkuympyräkatu 3 | Real estate | 2021 |