The fee is a record for a Spanish defender and the deal is the most expensive transfer in Bilbao’s history.
Martinez, linked to Barcelona this month, will replace Aymeric Laporte who has joined Manchester City and his signing continued Bilbao's policy of only recruiting players born in the Basque region.
The Spain international is renowned for scoring spectacular goals and netted his first senior goal against Bilbao from inside his own half in his debut season.
Martinez had previously said he would never join Athletic.
“I don’t think they’d ever come here and give their rivals 30 million euros (£26.4 million). I’m not thinking in this at all, and besides I’d never go to the other side and I’m relaxed about that,” the 26-year-old said when a move was mooted in 2014.
(Reporting by Joseph Cassinelli, editing by Ed Osmond)
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