Additions
| from | Name | Pos | Team | Team | ||
|---|---|---|---|---|---|---|
| 10/2026 |  | Tasio | GK | |||
| 09/2026 | | Asier Cerrillo | GK | |||
| 09/2026 | | Asier Torres | FW | |||
| 07/2026 | | Zugatz | GK |  | Athletic Club B | Bilbao B |
| 07/2026 |  | Aitor León | DF |  | Athletic Club U19 | Bilbao U19 |
| 07/2026 | | Mikel | DF | | Athletic Club B | Bilbao B |
| 07/2026 | | Onukwuru | DF | |||
| 07/2026 | | Pablo Martínez | DF | | Athletic Club B | Bilbao B |
| 07/2026 | | Urko | DF | | Athletic Club B | Bilbao B |
| 07/2026 | | Iñigo Sainz | MF | | Athletic Club U19 | Bilbao U19 |
| 07/2026 | | Unax | MF | | Athletic Club B | Bilbao B |
| 07/2026 | | Aitor Torrico | FW |  | SD Amorebieta | SD Amorebieta |
| 07/2026 | | Danel Narvarte | FW | | Athletic Club U19 | Bilbao U19 |
| 07/2026 | | Ismahel Saidani | FW | | Athletic Club U19 | Bilbao U19 |
| 07/2026 | | Xabi Gómez | FW |  | Club Portugalete | Portugalete |
| 07/2026 |  | Xantiago | FW | | Athletic Club B | Bilbao B |
| 07/2026 | | Adrián Esteve | MA |
Departures
| from | Name | Pos | Team | Team | ||
|---|---|---|---|---|---|---|
| 07/2026 | | Eric | GK |  | UD Logroñés Promesas | UD Logroñés Promesas |
| 07/2026 | | Iker Pagazartundua | GK | | Athletic Club B | Bilbao B |
| 07/2026 | | Simón García | GK | | Athletic Club B | Bilbao B |
| 07/2026 | | Beñat Larrea | DF | | Athletic Club B | Bilbao B |
| 07/2026 | | Danel Belategi | DF | | Athletic Club B | Bilbao B |
| 07/2026 | | Daniel Pérez | DF | | Athletic Club B | Bilbao B |
| 07/2026 | | David Osipov | DF | | UD Logroñés Promesas | UD Logroñés Promesas |
| 07/2026 | | Zarandona | DF | | Athletic Club B | Bilbao B |
| 07/2026 | | Eneko Ellakuria | MF | | Athletic Club B | Bilbao B |
| 07/2026 | | Selton Sánchez | MF | | Athletic Club B | Bilbao B |
| 07/2026 | | Ander Peciña | FW | | Athletic Club B | Bilbao B |
| 07/2026 | | Aritz Conde | FW | | Athletic Club B | Bilbao B |
| 07/2026 | | Txus Vizcay | FW | | Athletic Club B | Bilbao B |
| 07/2026 | | Bittor Llopis | MA | | Athletic Club B | Bilbao B |