The 23-year-old Spaniard has signed a three-and-a-half-year deal with the Championship outfit after Norwich accepted an undisclosed fee.
Ayala, a former Liverpool trainee, joined Middlesbrough on loan in October and has made 13 appearances for the club so far.
Having mustered only 10 first-team outings in an injury-hit first campaign with Norwich following his arrival from Liverpool in August 2011, Ayala was loaned to Nottingham Forest last season.
He once again struggled for fitness at the City Ground and appeared in only 14 games.
However, Ayala has been able to feature regularly for Aitor Karanka's Middlesbrough, who are seeking a return to England's top flight.
'People will remember us because we got to No.1 in the world rankings and achieved great things - but our biggest regret is that we didn’t win anything': Ex-Everton star Kevin Mirallas opens up on 'honour' of being part of nation's golden generation
‘I’m sure we’d have finished in the top four or five if he hadn’t joined Chelsea in January. Him leaving damaged the team, but we couldn’t stand in his way’: Alan Curbishley reflects on failed Champions League dream after selling key player
