The Queens Park Rangers defender is close to a loan move to St Mary's.
The 23-year-old endured a disappointing campaign at Loftus Road last season but Southampton are keen to add a defender to their ranks after losing Toby Alderweireld.
Indeed, Sky Sports suggest Ronald Koeman has been on the lookout for one more defensive recruit and that Caulker may now fill that void.
Are Southampton making a mistake?
The Saints are clearly in need of a new centre-back after Alderweireld's departure but QPR's Caulker may well end up disappointing the South Coast side.
Chris Ramsey has taken a good hard look at his R's squad and it is no surprise he is willing to let the 23-year-old go. Those he has deemed worth keeping have so far stayed.
Tony Fernandes, meanwhile, announced on Tuesday afternoon that some QPR players will leave if their valuation is met.
Caulker, however, is likely someone deemed surplus to requirements rather than the subject of an astronomic Southampton offer. Cardiff took the defender off Tottenham's hands and he showed promise. But his Premier League campaign last time out was a real disappointment; he received plenty of criticism for his displays after joining QPR.
If the Saints simply need a defender for back-up, then, Caulker may work out. But an Alderweireld replacement he certainly is not.