Athletic Bilbao and Real Sociedad are Basque rivals, although there’s not as much bad blood, so as to speak, between the two clubs as there would be, between say, Lazio and Roma, or Rangers and Celtic.
Somewhat unique in this respect, Athletic Club have the policy of signing only Basque players. Sociedad used to follow that policy too before abandoning it in the late 1980’s when they signed John Aldridge from Liverpool.
Following Inigo Martinez’s transfer from Sociedad to Bilbao for 32 million euros (also now the most expensive Spanish defender in history), here is a list of famous players to have transferred from La Real to Athletic Club. Before Martinez, the biggest transfer was that of Joseba Etxeberria, who moved from the Anoeta to San Mames in 1995.
List of major transfers between Athletic Club and Real Sociedad (Basque derby rivals)
Inigo Martinez (From Real Sociedad to Athletic Bilbao 2018)
Gorka Elustondo (From Real Sociedad to Athletic Bilbao 2015)
Xabi Castillo (From Real Sociedad to Athletic Bilbao 2009)