Cardiff U21: every transfer in and out, fees paid and earned, squad market value.
| Date | Player | From | To | Fee |
|---|---|---|---|---|
| 2025-07-01 | Ronan Kpakio | Cardiff U21 | Cardiff | — |
| 2025-07-01 | Joel Colwill | Cardiff U21 | Cardiff | — |
| 2025-07-01 | Dylan Lawlor | Cardiff U21 | Cardiff | — |
| 2025-01-12 | Joel Colwill | Cardiff U21 | Exeter City | Loan |
| 2024-08-02 | Joel Colwill | Cardiff U21 | Cheltenham | Loan |
| 2024-07-01 | Ronan Kpakio | Cardiff U18 | Cardiff U21 | — |
| 2024-07-01 | Dylan Lawlor | Cardiff U18 | Cardiff U21 | — |
| 2023-07-26 | Tom Davies | Cardiff U21 | Kilmarnock FC | Loan |
| 2023-07-01 | Eli King | Cardiff U21 | Cardiff | — |
| 2023-01-31 | Tom Davies | Cardiff U21 | Pontypridd | Loan |
| 2022-08-25 | Eli King | Cardiff U21 | Crewe Alexandra | Loan |
| 2022-07-01 | Joel Colwill | Cardiff U18 | Cardiff U21 | — |
| 2021-08-30 | Roland Idowu | Cardiff City U23 | Bohemians | Free |
| 2021-07-01 | Isaak Davies | Cardiff City U23 | Cardiff | — |
| 2021-07-01 | Eli King | Cardiff U18 | Cardiff U21 | — |
| 2020-09-21 | Roland Idowu | Southampton U18 | Cardiff City U23 | Free |
| 2020-07-01 | Mark Harris | Cardiff City U23 | Cardiff | — |
| 2020-07-01 | Isaak Davies | Cardiff U18 | Cardiff City U23 | — |
| 2019-07-01 | Mark Harris | Cardiff City U23 | Wrexham | Loan |
| 2019-01-23 | Mark Harris | Cardiff City U23 | Port Vale | Loan |