The defender will be returning on loan from Celta Vigo, while youngster Unai Egiluz has been loaned out to CD Mirandés.
Athletic Bilbao coach Ernesto Valverde has welcomed the return of Unai Nuñez.AdvertisementAdvertisementValverde said: "It's a positive move. Unai's a player with experienced and, bearing in mind that we'll play more matches this season, he could be good for us.
"What's more, it allows Egiluz to go out on loan and continue gaining experience.
"We believe these were good deals for both the players and for us."