After a lethargic first half, Luke Leahy capitalised on some sloppy defending early in the second to score for the visitors.
But Pitman confidently netted a penalty following a foul on Nicke Kabamba and it was the Blues who ended the contest on top.
Kenny Jackett made one change from the side beaten at Oxford in the last league outing.
Milan Lalkovic had recovered from an ankle injury to feature against his former club, with Kyle Bennett dropping to the bench.
The 4-2-3-1 system remained, although this time it was skipper Pitman up front, with Kal Naismith just behind.
New signing Dion Donohue had to watch from the stands after completing his Fratton switch too late to feature.
Before kick-off there was a minute’s silence in memory of all those who lost their lives in the Barcelona tragedy.
Once the action got underway, referee Brett Huxtable soon had to reach for his pocket, showing Saddlers skipper Adam Chambers a yellow card following a late challenge on Adam May.
But it was the visitors who looked brighter in the opening stages and Leahy unleashed a fierce shot that flew narrowly past the post.
The wing-back then missed a great opportunity to break the deadlock on 15 minutes, heading wide from close range after good work down the right by James Wilson.
Christian Burgess came to the rescue moments later when a sloppy Jack Whatmough pass gifted possession to Amadou Bakayoko.
But it was Whatmough – celebrating his 21st birthday – who then started the move that led to Pompey’s best chance of the half on 31 minutes.
The defender’s superb cross-field ball picked out Gareth Evans on the right and his cross was met by the head of a diving Pitman, only for Shaun Donnellan to make a vital block.
Evans then sent a header over the bar from Danny Rose’s cross, as the hosts finally started to pick up the pace.
But there was a blow for the Blues on 36 minutes, as Drew Talbot became the latest victim of the defensive injury curse.
He went down clutching his hamstring and limped to the sidelines, with Bennett coming on to make his 100th appearance.
The remainder of the half did little to excite a large Fratton crowd, though, with the half-time whistle greeted by yawns rather than cheers.
It was pretty sleepy stuff before the break and Pompey clearly had not woken up for the start of the second half.
Wilson sent in a low cross from wide on the right and Leahy was left all alone to tap home at the far post.
The Blues were struggling to create much at the other end of the pitch, as Mark Gillespie continued to enjoy a comfortable afternoon between the sticks.
He was not tested by Bennett on the hour mark, with the substitute firing wide from the edge of the box after being teed-up by Naismith.
Jackett opted to make a double substitution on 65 minutes, as Naismith and Lalkovic made way for Curtis Main and Kabamba.
But Walsall almost doubled their lead soon after, with Luke McGee saving well from Amadou Bakayoko and then Burgess blocking a shot from the same player.
The Blues were presented with an excellent chance to level on 74 minutes, though, when Gillespie tripped Kabamba, who did not appear to be posing a threat.
Huxtable immediately pointed to the spot and PITMAN made no mistake, slamming home his third goal of the season.
That provided a spark for Pompey and they immediately started to display more energy going forward.
Pitman almost grabbed his second with a shot that dipped narrowly over the bar after Bennett had done well to steal possession.
It was the Blues looking more likely to grab a winner and Pitman in particular was looking a threat, not quite able to guide Adam May’s pass on target on 87 minutes.
But, despite five minutes being added on for stoppages, Pompey were not able to complete the turnaround by finding a winner.
Pompey (4-2-3-1): McGee; Talbot (Bennett 36), Burgess, Whatmough, Haunstrup; Rose, May; Evans, Naismith (Kabamba 65), Lalkovic (Main 65); Pitman (c)
Goals: Pitman 74 (pen)
Subs not used: Bass, Davies, Close, Chaplin
Walsall (3-5-2): Gillespie; Wilson, Donnellan, Guthrie; Devlin, Morris (Cuvelier 69), Chambers (c), Edwards, Leahy; Oztumer, Bakayoko (Jackson 77)
Goals: Leahy 47
Booked: Chambers, Guthrie, Devlin, Bakayoko
Subs not used: L.Roberts, K.Roberts, Kouhyar, Kinsella, Candlin
Referee: Brett Huxtable
Attendance: 17,198 (464 away fans)