Karanka has asked for a mutual termination, which has been granted. He has not been sacked.
The Spaniard was not happy with outside influences that disrupted his job, while results were not what he had expected either - per Sky Sports News.
Forest are seventh in the Championship, only four points away from a playoff spot.
Karanka felt as though the only ones at the club who supported him were the fans, making his job almost impossible.
"The club have agreed to this request and the terms of departure have been agreed. Both parties wish each other every success for the future.
"There will be no further comment from either party.
"Simon Ireland will take charge of first-team affairs until a new manager is appointed."