The 25-year-old officially put pen to paper on Monday, with his new contract due to run until 2019.
"The more years I could sign for, all the better it was for me. Five years is an important guarantee for me. It shows the club backs me and I need to show them that they've made the right decision," Inside Spanish Football quotes De Marcos as saying.
"I've always wanted to be here, so long as the club wants me."
De Marcos has made just short of 200 appearances for the Basque side since moving from Deportivo Alaves in 2009.