Ibrahima Diallo has made the move to the south coast from French side Brest on a four-year deal.
The 21-year old midfielder is believed to have cost around £11 million as the Saints look to strengthen their squad for the 20/21 Premier League campaign.
Diallo initially broke through the Monaco academy before signing for Brest in 2019 following a successful loan period.
In the 2018/19 season, he made 28 appearances as Brest successfully won promotion from Ligue 2 into the top flight of French football.
Since then the defensive midfielder has gone on to regularly feature for the French side, including in the opening fixtures of the new campaign.
Diallo has also been included in the French Under-21 squad, although has yet to debut after most recently sitting on the bench in April 2020.
"He is an important signing for us"
Ralph Hasenhuttl spoke highly of the Frenchman after he signed by saying:
"I am very happy he will come to us and hopefully he will have a good time with us"
"He's a very smart one, a very clever one, he reads the game very well. He's also quick and dynamic."
"He is an important signing for us, because in midfield we only have so many alternatives, and he showed in France that he is a fantastic midfield player. He is a player who I think fits perfectly in our team."