Forest are seventh in England's second tier, 12 points adrift of leaders Leeds United and four points off the playoff places.
A former Real Madrid and Athletic Bilbao player, Karanka joined Forest in January last year. The 45-year-old had previously been fired by then Premier League side Middlesbrough.
"Aitor Karanka has asked to be released from his contract as the manager of Nottingham Forest Football Club," Forest said in a statement.
"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."
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Forest added that first team coach Simon Ireland would take charge until a new manager was appointed.
(Reporting by Alan Baldwin; Editing by John O'Brien)
