The midfielder, who was at Chelsea for 11 seasons before leaving for Chinese club Tianjin Teda, left Teda by mutual consent, according to the Guardian.
Middlesbrough, who are currently 5th place in the 2nd tier, announced the signing on their social media page, sharing a video of Mikel Obi.
Play for the name on the front of the shirt - and they'll remember the name on the back ??@mikel_john_obi excited to be joining 'family club' #Boro ?? https://t.co/qvEO8laTOp #UTB pic.twitter.com/qOVUjtZiYi
