The club has released an official statement confirming the news.
Middlesbrough have parted ways with head coach Aitor Karanka, with Steve Agnew stepping in to take control of the first team.
The news was confirmed today, ahead of their League clash against Manchester United this Sunday.
The Premier League side are currently on 22 points in the relegation zone, having failed to win any of their last 10 games in the competition.
Karanka paid tribute to the club in a statement which was published on the club website:
"Middlesbrough Football Club can today confirm the departure of Head Coach Aitor Karanka. Following discussions, both parties have agreed that it is in the best interests of the club to make this change.
"The club would like to thank Aitor for all his hard work over the past three-and-a-half years and for the success we've had during that time.
"We wish him all the best for the future.
Aitor Karanka said: "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. 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."
"Steve Agnew will step up from his role as assistant Head Coach to take control of first team affairs with immediate effect."
Karanka, who was first appointed at Middlesbrough in 2013, led them to Premier League promotion last season.
The former Real Madrid assistant manager previously expressed his frustration at the lack of new signings during the January Transfer Window, and blamed fans for asking his team to play long-ball tactics following a defeat to West Ham.