| Isoler Ltd. | Construction | 1992 |
| Istanbul Barbers (Durham) Ltd | Other services | 2025 |
| Istanbul Shawarma Durham Ltd | Hospitality | 2025 |
| It's Bennies Ltd | Wholesale & retail | 2025 |
| It-Active Training Ltd | Information & communication | 2019 |
| Italian Fusion Ltd | Hospitality | 2026 |
| Italy Services Durham Limited | Hospitality | 2023 |
| Itelo Limited | Information & communication | 2006 |
| Ivan Simpson Bathrooms and Kitchens Limited | Wholesale & retail | 2022 |
| Ivron Technologies Ltd | Professional & scientific | 2026 |
| Izman UK Ltd | Health & social care | 2013 |
| Izsa Group Limited | Wholesale & retail | 2024 |
| J & D Car Sales Ltd | Wholesale & retail | 2018 |
| J & JJ Properties Ltd | Real estate | 2024 |
| J & L Welch Roofing & Paving Services Ltd | Administrative & support | 2021 |
| J & S Taylor (Site Services) Ltd | Administrative & support | 2003 |
| J a T Assist Limited | Extraterritorial | 2009 |
| J and B Nelson Construction LLP | | 2001 |
| J and D Carpets Ltd | Wholesale & retail | 2006 |
| J C Pearson Author Limited | Information & communication | 2019 |
| J Crowe Brickwork Ltd | Construction | 2026 |
| J D Winter Ltd | Construction | 2013 |
| J Fire Protection Ltd | Construction | 2015 |
| J K Transport Services (Durham) Ltd | Transport & logistics | 2023 |
| J L Durham Student Cleans Ltd | Administrative & support | 2019 |
| J Miller Property Ltd | Real estate | 2025 |
| J O Controls Ltd | Information & communication | 2014 |
| J P Quarm Investigations Ltd | Public administration | 2017 |
| J Rowntree Ltd | Construction | 2024 |
| J S Brown Properties Limited | Real estate | 2016 |
| J Tait Flooring Ltd | Construction | 2023 |
| J Thornton Construction Services Ltd | Construction | 2024 |
| J Turnbull Groundworks Ltd | Construction | 2014 |
| J W Steed Consulting Limited | Professional & scientific | 2011 |
| J W Wood (Darlington) Limited | Real estate | 1994 |
| J Wilson TM Services Ltd | Construction | 2024 |
| J&E Plant & Machinery Ltd | Administrative & support | 2017 |
| J&J Inspection Services Ltd | Mining & quarrying | 2026 |
| J&Jk Ltd | Professional & scientific | 2014 |
| J&K Property Rental (Durham) Limited | Real estate | 2026 |
| J. D. S. (City Developments) Limited | Real estate | 1981 |
| J.G. Paxton and Sons Group Limited | Manufacturing | 2026 |
| J.G. Paxton and Sons Limited | Manufacturing | 1983 |
| J.S. Ductair Services Limited | Construction | 1998 |
| J.T.E. Construction NE Ltd | Construction | 2018 |
| J.W. Wood Commercial Limited | Real estate | 1997 |
| J3 Group Ltd | Hospitality | 2026 |
| JA Property Limited | Real estate | 2001 |
| Jacaranda Imaging Limited | Health & social care | 2023 |
| Jack Wilson Performance Ltd | Wholesale & retail | 2025 |
| Jack-Pot HR Ltd | Administrative & support | 2025 |
| Jack.Codes Limited | Information & communication | 2016 |
| Jackdaw Design Ltd | Professional & scientific | 2025 |
| Jacks SIM Rooms Ltd | Construction | 2025 |
| Jackson North East Properties Ltd | Households | 2025 |
| Jacqueline Stanhope Fine Art Limited | Arts & recreation | 2012 |
| Jade Properties (Northern) Limited | Real estate | 2004 |
| JAE Construction Limited | Construction | 2026 |
| JAE Homes Ltd | Real estate | 2024 |
| JAG Consultancy Services Ltd | Information & communication | 2015 |
| JAM Scanning Ltd | Health & social care | 2017 |
| Jambh Services Ltd | Administrative & support | 2025 |
| Jambo Yoga Limited | Health & social care | 2011 |
| James & James Joinery Ltd | Construction | 2024 |
| James Hogg Podiatry Ltd | Health & social care | 2018 |
| James Kenny Studio Ltd | Professional & scientific | 2023 |
| James Lawton Homes (North Gosforth) Ltd | Construction | 2021 |
| James Towland Logistics Ltd | Transport & logistics | 2025 |
| Jamiechancetravels Ltd | Professional & scientific | 2025 |
| Jane J Designs Limited | Professional & scientific | 2015 |
| Janet Maitland (Bowburn) Limited | Other services | 2013 |
| Janet Maitland (Durham) Ltd | Other services | 2011 |
| Janet Maitland (Hetton) Ltd | Other services | 2011 |
| Janus Group Services Ltd | Information & communication | 2024 |
| Japanese Head Spa Durham Ltd | Other services | 2024 |
| Jawanda Couriers Ltd | Transport & logistics | 2026 |
| Jaxholme Holdings Limited | Financial services | 2022 |
| JAY Steele Offshore/Onshore Services Ltd | Professional & scientific | 2026 |
| Jaydeen Sarah Cosmetics Limited | Wholesale & retail | 2025 |
| Jayell (NE) Limited | Agriculture & fishing | 2014 |
| JB Equine Limited | Agriculture & fishing | 2012 |
| JB Groundworks (NE) Ltd | Construction | 2018 |
| JB Haulage Ltd | Transport & logistics | 2021 |
| Jb-Oh Limited | Other services | 2019 |
| JBA Kooleks Limited | Wholesale & retail | 2024 |
| JBD Systems Ltd | Wholesale & retail | 2020 |
| Jblf Technology Limited | Manufacturing | 2019 |
| JBR Transport 2020 Limited | Transport & logistics | 2020 |
| JBS Properties Limited | Real estate | 2015 |
| JC CNC Maintenance Ltd | Manufacturing | 2022 |
| JCF Construction Ltd | Construction | 2025 |
| Jcholmes Ltd | Wholesale & retail | 2022 |
| JCL Services Ltd | Real estate | 2020 |
| JCS Construction NE Ltd | Construction | 2025 |
| JCS Heating & Plumbing Ltd | Construction | 2022 |
| JD Body and Paint Ltd | Wholesale & retail | 2018 |
| JD Durham Ltd | Real estate | 2022 |
| JD Telecoms NE Limited | Construction | 2026 |
| Jdbproperty Ltd | Real estate | 2025 |
| Jdbtech Ltd | Information & communication | 2025 |