Athletic Bilbao have signed Inigo Martinez from Basque rivals Real Sociedad after meeting his €32 million release clause.
The Spain centre-back comes in after Aymeric Laporte joined Manchester City for €65m on Tuesday.
Martinez has agreed a contract until June 2023 with Athletic in a transfer that is bound to provoke controversy in the Basque region.
It is a case of third time lucky for Athletic, who previously failed in efforts to sign Asier Illarramendi and Mikel Oyarzabal from Real.
Martinez, 26, will be presented by Athletic on Tuesday and could make his debut when they face Girona in La Liga on Sunday.
Athletic are 12th in La Liga, while Real are just six points above the relegation zone in 15th.
The second Basque derby of the season takes place on April 29 at Anoeta, with Martinez likely to receive a rocky reception.
Athletic were held to a 0-0 draw by Real at San Mames in December.