Southampton officially announced that the team signed Brest midfielder Diallo.
Diallo passed Southampton's physical exam at the weekend, and he signed a four-year contract with the team.
Diallo is the third new aid introduced by Southampton this summer. According to the update of the German transfer market website, his transfer fee is 15 million euros.
Diallo, who is only 21 years old this year, was born in Tours. He is the younger brother of Paris Saint-Germain defender Diallo.
Diallo joined the Monaco youth academy in 2014. He joined Brest on loan in the summer of 2018, after which he was officially bought out.
Diallo helped Brest advance to Ligue 1 and he played 5 times in Ligue 1 this season.