Head coach Aitor Karanka has left Middlesbrough by mutual consent, the club have announced.
The Spaniard spent three-and-a-half years at the Riverside Stadium, leading them to promotion to the Premier League last season and overseeing their first top-flight campaign since 2008/09 this term.
He departs with Boro sat 19th in the table.
"I'd like to thank Middlesbrough for a wonderful opportunity and the players, staff and all the people at the club who I have worked with," Karanka told mfc.co.uk.
"I'd also like to thank the fans for their support. This club will always hold a special place for me and I wish everyone connected with Middlesbrough Football Club the very best for the future."
Assistant head coach Steve Agnew will take charge of the first team with immediate effect, starting with Manchester United's visit on Sunday.