Monaco confirmed the appointment of Moreno and Jardim’s sack in a statement released on their official website.
Moreno, 42, signed a two and a half year deal until June 2022 and will take up his duties early next week.
Moreno led Spain six times and qualified the team for Euro 2020.
He has won the 2015 Champions League, two Spanish championships, three Spanish Cups, one Spanish Supercup and one Club World Cup with Barcelona (2014-2017), alongside Luis Enrique, whom he also assisted at AS Roma (2011-12) and Celta Vigo (2013-14).
This season, Onyekuru has made only four appearances in the league under Jardim, who led Monaco to Ligue 1 title and reaching the Champions League semi-finals during the 2016-17 campaign.
Jardim was earlier sacked and replaced by Thierry Henry before coming back to take over from the Arsenal legend who was also fired.
His last game in charge was a 5-1 home win against Lille on December 21.
Monaco are currently seventh on 28 points in the league table.