Diatta brings to an end a three-year stay at the Jan Breydel stadium, having joined from Sarpsborg in January 2018.
The 21-year-old won two league titles and one Belgium Super Cup with Club Brugge.
Diatta scored 10 goals in 23 appearances for Club Brugge in the first part of the 2020/2021 season.
Monaco, who are managed by former Bayern Munich manager, Niko Kovac, are currently fourth on the Ligue 1 table.
“I am very happy to get involved with AS Monaco and to have the opportunity to progress in a recognized and ambitious club in the French championship whose history everyone knows,” Diatta told the club website following his arrival at the Stade Louis II.
“I look forward to discovering this new environment and working hard with my new teammates to give the club back the confidence it has placed in me.”
Monaco sporting director Paul Mitchell added: “Like his qualities of percussion, dynamism, his skills in finishing but also his versatility and his international experience despite his young age, Krepin brings together many assets that will fit into the modern style of play advocated by Niko.
“The arrival of Krepin is part of the club's desire to further raise the quality level of the group for this season and to continue to build on the long term.”
Monaco has been capped 10 times by the Senegal national team and was named young player of the tournament at the 2019 Africa Cup of Nations.