The 21-year-old centre-back, who has represented France at underage level, has committed himself to les Rouges et Blanc until 2017 following his move from Valenciennes
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"This is really a big thrill for me to get involved with Monaco," Isimat-Mirin told the club's official website.
"I feel a lot of joy, but also great anticipation. I'm looking forward to Monday for the resumption of training.
"This team's project is fantastic and I will give everything to prove that I deserve to be part of it."
Monaco have been very busy in the transfer window already, having already made a number of notable signings, including that of Radamel Falcao from Atletico Madrid.