The Blues were due to host the struggling Shrimpers at Fratton Park tomorrow.
However, Kenny Jackett’s side opted to call off the game after Ronan Curtis (Republic of Ireland), Craig MacGillivray (Scotland) and Ross McCrorie were called up for international duty.
The new date has been set for Tuesday, November 5 (7.45pm kick-off).
Pompey were due to play Norwich under-21s in the EFL Trophy that evening.
However, that game’s been moved forward to Saturday, September 14 after Bury were expelled from the Football League.
A date for the Blues’ fixture against Rotherham – postponed because of Victorious Festival on Saturday, August 24 – is still to be decided.
Tickets for the Southend game are on sale now, while those purchased for the original date will remain valid.