Athletic are paying 15m up front plus 5m in bonuses for Djalo.
The deal was announced this morning as Athletic are publicly listed on the local stock exchange and so must make clear such investments.
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Djalo has been likened to Athletic star Nico Williams, though they insist he will not arrive as Williams' replacement.
With this deal, Djalo becomes the most expensive signing in Athletic's history.
Djal, 24, arrived in Bizkaia at three months old and was coached at UD San Miguel de Basauri and SD Begoa in Bilbao until at the age of 17 when he left for Braga,