The Saints, who were relegated to the Championship last term, secured the signing of 21-year-old Taylor Harwood-Bellis on deadline day, signing him on a season-long loan.
The centre-back is the second of star from Man City's academy that the former Premier League side have recruited this summer, with Under-21s captain Shea Charles joining for £15m too.
Southampton have also brought in Head of Academy Jason Wilcox to be their Director of Football and youngster Josh McNamara for their Under-21 team.
The Saints had the same policy this time last year, with them spending around £40m on Man City academy stars.
Romeo Lavia - who moved on to Chelsea this summer - Gavin Bazunu, Sam Edozie and Juan Larios all joined the Saints.
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Now, Southampton have decided to acknowledge they're evidently strong transfer relationship with the reigning Premier League champions.
Teasing the announcement of Harwood-Bellis, Southampton posted a picture of a loyalty card, with five circles marked out with a Man City stamp mark, leaving one final circle with 'bonus signing' written on it, before confirming the defender's arrival.
The England Under-21s star, who won the Under-21s European Championships this summer, played on loan at Burnley last term under former Man City captain Vincent Kompany.
He helped the Clarets achieve promotion back to the Premier League at the first time of asking, making 32 league appearances as they went up as champions.
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Meanwhile, Man City also announced the departure of striker Cole Palmer, with the academy graduate leaving for Chelsea for £45m.