"You can't imagine how delighted I am because it's not usual to bring players like him to recently promoted teams," head coach Aitor Karanka told the club's website (www.mfc.co.uk).
"I'm really pleased because he is really committed. All the conversations we've had he knows why he is here and he is coming to add to what we have."
The 34-year-old made over 500 appearances for Barcelona where he has won three Champions League trophies and six La Liga titles.
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Middlesbrough, who won promotion from the Championship last season, also signed defensive midfielder Marten De Roon, winger Viktor Fischer and defender Bernardo Espinosa.
(Reporting by Ian Rodricks in Bengaluru; editing by Clare Lovell)
