The Atletico Madrid defender looked set for a switch to Newcastle or Sunderland but the Saints have moved ahead of the North-East pair as deadline day approaches.
The 25-year-old had looked on course for a move to the North East, with the Magpies emerging as favorites to land the center back.
A late move for the Belgium international from Saints boss Ronald Koeman looks set to lure the defender to St Mary's Stadium.
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Arsenal, Liverpool and Tottenham have all been strongly linked with the former Ajax man, who is now set to seal a move to the Premier League.
Alderweireld moved to Atletico Madrid from Ajax last summer but made just 12 league appearances in the Spanish side's run to the Primera Division title.
The 25-year-old started all four of Belgium's games at the World Cup.