Mikel joins the English club in the January transfer window from Tianjin TEDA, the Chinese club.
The Nigerian captain rose to prominence in an 11-year stint with Chelsea, making nearly 400 appearances for The Blues and winning a host of honours, including two Premier League titles and the Champions League.
According to local media reports, Mikel becomes the fifth Nigerian to play for Middlesbrough, after Yakubu Aiyegbeni, Bart Ogbeche, Karl Ikeme and Kenneth Omeruo, the reports said.
The reports added that the Nigerian captain has battled injuries since the 2018 FIFA World Cup in Russia and has put in an effort to get back in top shape.
Middlesbrough are currently in the battle to gain promotion to the Premier League from the Championship.