"Nottingham Forest can confirm that manager Martin O'Neill has left the club," Forest announced in a brief statement.
The club then said 47-year-old former France international Lamouchi, who took Rennes from the relegation zone to Europa League qualification in the 2017-18 season, was taking charge.
"Forest are delighted to announce that Sabri Lamouchi has been appointed as the club's new head coach... the Frenchman will be assisted by six new members of staff who will support the first team," said a statement.
Lamouchi brings plenty of experience to the role, having led Ivory Coast to the 2014 World Cup as the top-ranked African side and the 2013 African Cup of Nations quarter-finals.
He also won the Qatari Cup and reached the Asian Champions League semi-finals during his time at El Jaish SC.
Lamouchi played for clubs AS Monaco, Parma, Inter Milan and Olympique Marseille as well as making 12 appearances for France.
O'Neill, who played 371 times for Forest from 1971-1981 and won two European Cups under Brian Clough, ended his spell as Ireland manager last November before moving to the City Ground in January.
The 67-year-old Northern Irishman O'Neill had replaced Spaniard Aitor Karanka and brought in former Forest midfielder Keane to assist him.
The pair had spent five years together with Ireland before they left their roles last November having won only once in 11 games and being relegated from the UEFA Nations League B.
However, former Manchester United captain Keane's tenure at Forest did not last long as he announced his departure on Sunday.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Ken Ferris)
