The Portuguese coach, appointed midway through the 2023/24 season, had been under contract until 2028 but has been relieved of duties.
His departure is expected to be confirmed imminently.
Forest survived relegation under Nuno last season and secured a surprise Europa League place, but the club’s form at the start of this campaign has been mixed -- with one win, one draw and one defeat from their opening three league games.
The team are due to face Porto and Braga in Europe, but Nuno will no longer be in charge for that return to continental competition.
Reports from The Athletic, the Portuguese newspaper Record and Fabrizio Romano indicated that the decision came from Marinakis, whose relationship with Nuno has become irreparably strained.
Nuno, a former goalkeeper, began his managerial career in 2012 and has since managed Rio Ave, Valencia, Porto, Wolves, Tottenham and Al-Ittihad in Saudi Arabia before arriving at the City Ground.