The Northern Irishman has parted ways with the Championship outfit four days after his assistant Roy Keane stepped away from his position.
Forest haven't wasted any time in replacing O'Neill, with former France global footballer Sabri Lamouchi already announced as their new boss - the 47-year-old having been sacked by Ligue 1 outfit Rennes last December.
Nearly immediately, the arrival in his place of former Rennes and Ivory Coast boss Lamouchi was announced.
In a brief statement released by the club this afternoon, they wished Martin O'Neill the best going forward, and thanked him "for all his efforts during his time at The City Ground".
Martin O'Neill has left his role as manager of Nottingham Forest.
O'Neill won two European Cups at the City Ground under Brian Clough but the reunion has soured and Forest are now looking for a new boss.
He led Ivory Coast to the 2014 World Cup and most recently managed Ligue 1 club Rennes for a season in 2017/18.
"The Frenchman will be assisted at Forest by six new members of staff who will support the first team".
Nottingham Forest is an opportunity for him to get back into the game and they will be aiming for the Play-Offs next season.