Moreno replaced Leonardo Jardim in December when Monaco were seventh in the table. He leaves with the team in ninth place, 28 points behind Paris St Germain, who were crowned champions when the French season was cut short by the COVID-19 pandemic.
"Our paths are separating earlier than expected but I would like to thank Robert Moreno for accepting the challenge," Monaco vice-president and general manager Oleg Petrov told the club website.
"With his staff, Robert did his utmost to improve the team, with enthusiasm and dedication. I wish him the best for the future."
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