| Iosym Limited | Manufacturing | 2000 |
| Irradiology Limited | Health & social care | 2024 |
| Isca Diagnostics Ltd | Professional & scientific | 2012 |
| Island View Capital Limited | Hospitality | 2022 |
| Island View House Ltd | Real estate | 2021 |
| Islands' Tourism and Business Partnership Limited | Other services | 2012 |
| ITT Construction Ltd | Construction | 2024 |
| IVY Farm Treen Limited | Real estate | 2014 |
| Izar Ltd | Wholesale & retail | 2017 |
| J & C Freeth Limited | Agriculture & fishing | 2014 |
| J & H Calland Limited | Hospitality | 2010 |
| J C Golf Academy Limited | Arts & recreation | 2024 |
| J C Plastics Limited | Wholesale & retail | 2003 |
| J C Williams Cars and Commercials Ltd | Wholesale & retail | 2023 |
| J Chanter Limited | Health & social care | 2009 |
| J Crowhurst Consulting Limited | Professional & scientific | 2013 |
| J H Events (Cornwall) Limited | Hospitality | 2023 |
| J Harris Construction (Cornwall) Ltd | Construction | 2025 |
| J Heathcote Masonry Ltd | Construction | 2019 |
| J Hull Bar Services Limited | Hospitality | 2023 |
| J K Motors (SW) Ltd | Wholesale & retail | 2016 |
| J Keane Plant Hire Limited | Construction | 2019 |
| J T Tiling and Bathroom Specialists Limited | Construction | 2006 |
| J V Richards (Rietfontein) Ltd | Agriculture & fishing | 1971 |
| J W Wealth Management Limited | Financial services | 2024 |
| J Ward Capital Ltd | Professional & scientific | 2026 |
| J&G Onda LLP | | 2024 |
| J&M a Limited | Wholesale & retail | 2025 |
| J&R Properties Group Limited | Real estate | 2026 |
| J. & B. Wootton Limited | Real estate | 1961 |
| J.D. Scaffolding (SW) Ltd | Construction | 2025 |
| J.H. & P.M. Hall (Earthmovers) Limited | | 1975 |
| J.L Surface Preparation Ltd | Manufacturing | 2022 |
| J.Zeluk PL Builders Ltd | Construction | 2020 |
| JAB Site Management Limited | Construction | 2012 |
| JAC (SW) Limited | Construction | 2014 |
| Jack Nowell 14 Limited | Arts & recreation | 2014 |
| Jack Richards Electrical Ltd | Construction | 2017 |
| Jade IVY Holdings Ltd | Real estate | 2025 |
| Jairos Courier Service Ltd | Extraterritorial | 2024 |
| Jaki Properties Ltd | Construction | 2024 |
| JAM Media House Limited | Financial services | 2023 |
| Jambo Limited | Administrative & support | 2006 |
| Jambo Property Limited | Hospitality | 2012 |
| James & Carmichael Consulting LLP | | 2014 |
| James Davis Special Effects Limited | Arts & recreation | 2015 |
| James Miller Financial Planning Limited | Financial services | 2010 |
| James Smith Designs Limited | Administrative & support | 2010 |
| Jamie Trounce Mini Digger Services Ltd | Construction | 2015 |
| Jane Wells Management Ltd | Administrative & support | 2016 |
| Jane's Jubilee Garage Limited | Wholesale & retail | 2012 |
| Janova Investments Ltd | Real estate | 2024 |
| JAS Financial Ltd | Professional & scientific | 2017 |
| Jason Bayly Limited | Construction | 2010 |
| Jason Bullen Limited | Construction | 2012 |
| Jason Thomas Performing Arts Limited | Education | 2019 |
| Jata Designs Ltd | Professional & scientific | 2023 |
| Jata Holdings Ltd | Financial services | 2023 |
| Jaten Ltd | Real estate | 2022 |
| Jaxsako Investments Limited | Real estate | 2020 |
| Jaxsako Ltd | Construction | 2016 |
| Jays' Wors and More Ltd | Manufacturing | 2025 |
| JB Electrical and Renewables Ltd | Construction | 2025 |
| JD Renewable Limited | Professional & scientific | 2021 |
| Jdig Limited | Professional & scientific | 2025 |
| Jdubz Conversions Ltd | Manufacturing | 2015 |
| Jenn's Diner Ltd | Hospitality | 2017 |
| Jenna Woolcock Ltd | Hospitality | 2023 |
| Jennings Construction Limited | Construction | 1999 |
| Jenny Spa Ltd | Other services | 2023 |
| Jess Collins Ltd | Professional & scientific | 2013 |
| Jess Ratty Consulting Ltd | Hospitality | 2012 |
| Jessie Stephenson Limited | Information & communication | 2025 |
| Jewell Construction Limited | Construction | 2012 |
| JFG Anaesthetics Limited | Health & social care | 2019 |
| JG Industries Ltd | Construction | 2026 |
| JH Catering (Cornwall) Limited | Hospitality | 2023 |
| Jian Foods Limited | Hospitality | 2024 |
| Jindalee Limited | Transport & logistics | 2018 |
| Jjrc Limited | Real estate | 2020 |
| JJW Land & Property Ltd | Real estate | 2023 |
| JL Cleaning SW Ltd | Administrative & support | 2024 |
| JLD Plant Hire Limited | Construction | 1999 |
| JLM Drivers Ltd | Transport & logistics | 2022 |
| JLN Associates Limited | Other services | 2010 |
| Jmodelmgmt Limited | Administrative & support | 2023 |
| JN Emmett Ltd | Information & communication | 2017 |
| Joanna Lucy Wood Limited | Extraterritorial | 2025 |
| Joanne Aromatherapy Limited | Wholesale & retail | 2025 |
| Joel Thomas Landscaping Limited | Administrative & support | 2026 |
| John Courage (Cornwall) Limited | Real estate | 1972 |
| John Hurle & Sons Limited | Agriculture & fishing | 2004 |
| John Powell Consultancy Limited | Professional & scientific | 2012 |
| John Savage Accountancy Ltd | Professional & scientific | 2015 |
| Johns & Sons Developments Limited | Construction | 2021 |
| JON Tremaine Ltd | Arts & recreation | 2012 |
| Jonathan Cowie Endodontics Limited | Health & social care | 2014 |
| Jonathan Lamb Limited | Arts & recreation | 2003 |
| Jones & Vasey Ltd | Construction | 2023 |
| Jopro Ltd | Professional & scientific | 2020 |