The teenage defender has moved to St Mary's on a permanent three-year contract, and will initially join up with the club's B team
Chelsea starlet Dynel Simeu has completed a £1.5 million move to Southampton ahead of the 2021-22 season.
Goal reported that the two clubs reached an agreement on the permanent transfer last week, and the deal has now been made official.
The 19-year-old has joined Southampton on a three-season deal, which will keep him at St Mary's until 2024, bringing to an end his seven-year affiliation with the Blues.