The 23-year old Spaniard joined the Teessiders on loan in October but has impressed so much at the Riverside Stadium that manager Aitor Karanka offered him a permanent contract.
Ayala has penned a three-and-a-half-year deal with Boro after making 13 Championship appearances.
Ayala started his career in England with Liverpool, signing a senior contract in 2009 after spending time in the academy, before moving to Norwich in 2011. During that period, he had loans with Hull City and Derby County while also having a short-term stay with Nottingham Forest in 2012/13 when a Canaries player.
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