Kocaelispor (Super Lig): every transfer in and out, fees paid and earned, squad market value, €2.4m spent and €1.6m earned.
| Date | Player | From | To | Fee |
|---|---|---|---|---|
| 2026-07-01 | Uğur Kaan Yıldız | Göztepe | Kocaelispor | Free |
| 2026-07-01 | Serdar Dursun | Kocaelispor | Without Club | — |
| 2026-07-01 | Habib Keïta | Clermont Foot | Kocaelispor | €700k |
| 2026-07-01 | Gökhan Değirmenci | Kocaelispor | Without Club | — |
| 2026-07-01 | Ahmet Oğuz | Kocaelispor | Without Club | — |
| 2026-05-16 | Ahmet Sağat | Kocaelispor | Without Club | — |
| 2026-03-03 | Arda Özyar | Kocaelispor U19 | Kocaelispor | — |
| 2026-02-22 | Deniz Ceylan | Kocaelispor U19 | Kocaelispor | — |
| 2026-01-31 | Mahamadou Susoho | Man City U21 | Kocaelispor | €400k |
| 2025-09-12 | Habib Keïta | Clermont Foot | Kocaelispor | Loan |
| 2025-09-11 | Serdar Dursun | Persepolis FC | Kocaelispor | Free |
| 2025-09-07 | Botond Balogh | Parma | Kocaelispor | Loan |
| 2025-09-06 | Hrvoje Smolcic | Frankfurt | Kocaelispor | Loan |
| 2025-09-01 | Darko Churlinov | Burnley | Kocaelispor | — |
| 2025-08-22 | Tayfur Bingöl | Besiktas | Kocaelispor | — |
| 2025-08-14 | Karol Linetty | Torino | Kocaelispor | Free |
| 2025-08-13 | Can Keleş | Besiktas | Kocaelispor | Loan |
| 2025-08-01 | Serhat Öztaşdelen | Altinordu | Kocaelispor | €124k |
| 2025-07-30 | Mateusz Wieteska | Cagliari | Kocaelispor | Loan |
| 2025-07-25 | Anfernee Dijksteel | Middlesbrough | Kocaelispor | Free |