Lee Bradbury marked his return to Portsmouth from Crystal Palace with a fierce strike as Walsall collapsed in disarray at Fratton Park.
Walsall were level at the interval but fell away after the break, conceding two penalties and substituting their goalkeeper as Portsmouth ran riot.
Walsall began brightly enough and took the lead in the 10th minute with a stunning free-kick from 20 yards by midfield player Darren Wrack.
Wrack stepped up to bend a right-foot shot around the wall and in off the bar.
Portsmouth soon gained the initiative and equalised in first-half injury time.
Fitzroy Simpson took a corner on the right and Danish midfield player Thomas Thogersen scored with a header from 10 yards, although Walsall claimed Neil Poynton had hacked it off the line.
There was no mistaking Portsmouth's mastery after the break but it was not until the 69th minute that they got the goal their pressure deserved.
Simpson took advantage of a slip by Chris Marsh to set up a shooting chance which Bradbury put away with a fierce shot from 12 yards.
Walsall lost all their defensive discipline after Bradbury's goal and conceded two penalties in four minutes, converted both times by Alan McLoughlin against different goalkeepers.
In the 75th minute Bradbury was brought down by Marsh and Adrian Viveash, and McLoughlin beat James Walker with the spot-kick.
Walsall manager Ray Graydon substituted Walker after the goal because he had been booked for protesting at the award.
Four minutes later his replacement Carl Emberson was also beaten by McLoughlin from the spot after Ian Roper had handled under pressure from Bradbury.
Graydon made a triple substitution in a bid to stem the flow but there was still another goal to come in the 79th minute.
McLoughlin was this time the provider and although Thogersen missed his first kick he scored with his second from six yards.
Portsmouth manager Alan Ball said: "Lee Bradbury did very well on his debut, leading the line well and giving us a bit of a cutting edge. We recovered from a poor start and gave the fans something to cheer about."