David Moyes' men have secured the signature of their former midfielder on a six-year contract after agreeing a deal with Real Madrid.
Asier Illarramendi has completed his return to Real Sociedad, just two years after leaving the club for Real Madrid.
The 25-year-old has signed a six-year deal with David Moyes' side after passing a medical on Wednesday and will be officially unveiled Wednesday.
A statement on La Real's website read: "Real Sociedad and Real Madrid have reached an agreement for the transfer of Asier Illarramendi to our club.