Brazil goalkeeper Julio Cesar has joined Benfica on a two-year deal after being released by QPR.
Fox has sunk down the pecking order at St Mary's following the emergence of England Under-21 left-back Luke Shaw.
Now Bolton have made a move to land Fox, who has featured just once for the Saints this term.
Fox will join Bolton until January, when they could opt to make the move permanent.
Fox is set to be the latest loan recruit for Bolton, who snapped up Liam Feeney from Millwall, Neil Dann from Leicester and Cardiff's Kevin McNaughton last week.