The 31-year-old, who won the Champions League with Chelsea in 2012, was a free agent after leaving Chinese side Tianjin Teda last month.

The Nigeria midfielder made 249 Premier League appearances for Chelsea in an 11-year spell which ended when he left Stamford Bridge two years ago.

He also won two league titles, three FA Cups and the Europa League in 2013.

"I'm pleased to get him in. He is a man with a lot of experience and quality," Boro manger Tony Pulis told the club website.

"He has a winning mentality and he wants to be here to be a part of what we are looking to achieve."

Mikel has won a total of 85 caps for Nigeria, playing for them in the last two World Cups and helping the Super Eagles win the 2013 Africa Cup of Nations.

He was also one of Nigeria's three over-age players as the African side won a bronze medal at the 2016 Olympics in Rio.