| Iguana Apparel LLP | | 2014 |
| Iguana Sports Group Ltd | Wholesale & retail | 2024 |
| ILA Properties Limited | Real estate | 2018 |
| Ilocandia Co. Ltd | Health & social care | 2021 |
| Imperial Apex Designs Ltd | Construction | 2025 |
| Infinity Teaching Ltd | Education | 2013 |
| Infinity-Streaming Limited | Information & communication | 2020 |
| Initial Car Sales Limited | Wholesale & retail | 2025 |
| Initial Construction Services Limited | Construction | 2016 |
| Ink-Cognito Cardiff Limited | Arts & recreation | 2013 |
| Inspire Genius Foundation | Education | 2015 |
| Instock Commerce Ltd | Wholesale & retail | 2026 |
| Interior D:Zine Limited | Wholesale & retail | 2014 |
| International Student Club Limited | Education | 1997 |
| IOS Investments Ltd | Real estate | 2018 |
| Isaac Blake Ltd | Real estate | 2024 |
| Island View Caravan Park Limited | Hospitality | 1999 |
| Isset International | Education | 2021 |
| IT Solution Architecture Limited | Other services | 2019 |
| Italian Lighting Centre Ltd | Wholesale & retail | 2009 |
| Itch Investors Ltd | Professional & scientific | 2023 |
| ITF Wales Limited | Arts & recreation | 2004 |
| J & D Electrical Limited | Construction | 2015 |
| J & K (Cardiff) Limited | Real estate | 2009 |
| J C Mills Properties Limited | Real estate | 2024 |
| J H Plant Hire & Sales Limited | Administrative & support | 2021 |
| J Wilson Design Ltd | Professional & scientific | 2025 |
| J&B Trading Ltd | Wholesale & retail | 2021 |
| Jack Dust Ltd | Wholesale & retail | 2024 |
| Jadd Scaffolding Ltd | Construction | 2020 |
| JAG Hairdressers Limited | Other services | 2010 |
| James Atelier Ltd | Professional & scientific | 2026 |
| James Cockfield Limited | Information & communication | 2022 |
| JAR Property Holdings Limited | Households | 2024 |
| Jatayu Estates Ltd | Real estate | 2026 |
| JBE Health Training Ltd | Health & social care | 2015 |
| Jbgeological Services Limited | Mining & quarrying | 2014 |
| JDJ Properties Ltd | Real estate | 2020 |
| JET Engine Management Limited | Information & communication | 2008 |
| JF Urology Ltd | Health & social care | 2021 |
| JG Care & Supplies Limited | Wholesale & retail | 2019 |
| JH Demolition and Site Services Ltd | Construction | 2023 |
| JH Integrated Services Ltd | Households | 2025 |
| JJR (Retail Gifts) Limited | Wholesale & retail | 2017 |
| JMD Restaurants (Cardiff) Limited | Hospitality | 1999 |
| JMH Collaborations Ltd | Other services | 2017 |
| JMW Energy Services Limited | Energy | 2025 |
| JO Herbert Regulatory Consultancy Limited | Professional & scientific | 2013 |
| JO2WEB Limited | Information & communication | 2013 |
| John Hardy Music Limited | Information & communication | 2003 |
| JON Stock Agencies Ltd | Wholesale & retail | 2013 |
| Jonathan Popham Mortgage Services Limited | Financial services | 2024 |
| Jones & Thomas Developments Ltd | Construction | 2024 |
| JPD Carpentry Limited | Construction | 2019 |
| JPR Consultancy Limited | Arts & recreation | 2007 |
| JRP Ventures Limited | Real estate | 2022 |
| JRS Homes (Creigiau) Limited | Real estate | 2014 |
| JRW Housing Ltd | Real estate | 2016 |
| Jswg Holdings Ltd | Financial services | 2024 |
| Just File Accounts Limited | Information & communication | 2025 |
| Justfleets Ltd | Professional & scientific | 2025 |
| JW Electrical & Building Ltd | Construction | 2022 |
| JW1246 Consulting Ltd | Information & communication | 2023 |
| K & R Property Holdings Limited | Professional & scientific | 1998 |
| K Brows and Co Ltd | Other services | 2024 |
| K C J Properties (Cardiff) Limited | Real estate | 2026 |
| K5 Consulting Ltd | Professional & scientific | 2018 |
| Kabode Limited | Administrative & support | 2022 |
| Kaphri Consultants Limited | Real estate | 2000 |
| Kapitol Festivals Limited | Administrative & support | 2024 |
| Kapitol Media Limited | Information & communication | 2009 |
| Keem Bay Asset Management Limited | Real estate | 2014 |
| Keogh Property Partners Limited | Real estate | 2015 |
| Key Transformation Services Ltd | Professional & scientific | 2026 |
| Keycreate Community Interest Company | Other services | 2016 |
| KIT Lambert Ltd | Information & communication | 2018 |
| Kitchenology Limited | Wholesale & retail | 1949 |
| KJH Construction Ltd | Real estate | 2026 |
| Klappo Limited | Wholesale & retail | 2009 |
| KLH Accounting Limited | Professional & scientific | 2023 |
| Kootan Properties Limited | Real estate | 2020 |
| KP Nova Properties Limited | Real estate | 2025 |
| KS Notary Services Ltd | Professional & scientific | 2023 |
| KT & CP Trading Limited | Hospitality | 2016 |
| KTO Trading Limited | Other services | 2016 |
| Kumar's Kitchen Ltd | Hospitality | 2026 |
| Kura Software Limited | Information & communication | 2014 |
| L C G Construction Ltd | Construction | 2017 |
| L J R D Consultancy Limited | Financial services | 2015 |
| L R Holdings Limited | Real estate | 2015 |
| L&Kk Property Ltd | Real estate | 2020 |
| L&Lpub Ltd | Hospitality | 2026 |
| L&T Electrical Ltd | Construction | 2024 |
| Lancaster House (Wales) Limited | Real estate | 2014 |
| Landmarks Holding Ltd | Real estate | 2026 |
| Landmarkz Ltd | Wholesale & retail | 2024 |
| Lapider Limited | Professional & scientific | 2003 |
| Lasurgical Ltd | Health & social care | 2026 |
| Launch Packaging Design Limited | Professional & scientific | 2006 |
| Lavernock Service Station Ltd | Wholesale & retail | 2019 |