Middlesbrough head coach Aitor Karanka has appointed Steve Agnew as his number two following the departure of Craig Hignett earlier this month.
The 49-year-old joins Karanka's team from Premier League side Hull City where he had worked as assistant to manager Steve Bruce since 2012.
Agnew returns to the club where he previously served as first-team and academy coach between 2006 and 2012.
"We wish Steve all the best of luck," Hull boss Bruce told the club website.
"He has done a remarkable job as my assistant manager since 2012. Middlesbrough is a club close to his heart, his family are based in the area and so therefore we have reluctantly allowed him to move on."