Struggling Middlesbrough axe manager
Karanka paid the price for Middlesbrough's slump into the relegation zone after a miserable run of 10 league games without a win.
"Both parties have agreed that it is in the best interests of the club to make this change," a statement on Middlesbrough's website read.
"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."
Karanka's assistant Steve Agnew steps up to take control of first-team affairs with immediate effect.