The central midfielder has joined Saints on a four-year contract and the transfer fee is reported to be worth around £11million according to the BBC.
After Saints’ victory over West Brom, manager Ralph Hasenhuttl let slip that an announcement of a Diallo transfer was incoming.
The France Under-21 international is a product from the famed Monaco academy and joined Brest - the club Saints signed him from - initially on loan in Ligue 2.
Diallo helped Brest earn promotion to Ligue 1 and signed for the club on a permanent deal in 2019. He featured for the team 27 times last season before their campaign was cut short due to the COVID-19 coronavirus pandemic.
Saints’ new signing will only link up with his teammates after the international break after he was once again called up for the France U21s for their upcoming matches against Liechtenstein and Slovakia.
“We are happy to bring him in here,” Hasenhuttl said according to a club statement. “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 I am very happy that 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, against the ball very aggressive and with the ball also with good decision making, so he is a player who I think fits perfectly in our team.”
Southampton have been on the hunt for midfielder since former captain Pierre-Emile Hojbjerg left for Tottenham Hotspur earlier in the transfer window. Since then, Saints midfield pairing Harrison Reed and Mario Lemina left the South Coast to West London’s Fulham on a permanent deal and on loan respectively.
When does the summer transfer window open and close?
The summer transfer window opened on 27 July and closes at 23:00 on 5 October.
A domestic-only window for trades between Premier League and EFL clubs will run from 5 October until 16 October at 17:00.