Conor Shaughnessy headed home a late winner as Pompey secured the League One title on a dramatic night at Fratton Park.
It was a tense evening that understandably affected the performance on the pitch, with the hosts twice having to fight from behind against Barnsley.
Devante Cole’s early opener was quickly cancelled out by Kusini Yengi, only for John McAtee to strike again for the visiors after the break.
But Colby Bishop came off the bench to level from the spot and Shaughnessy converted a corner to send the home faithful wild with delight.
It meant that not only are the Blues heading up to the second tier, they are guaranteed to do so as worthy champions.
John Mousinho made five changes from the side that were held to a draw at fellow promotion hopefuls Bolton at the weekend.
Tino Anjorin was handed a first start since the FA Cup defeat at Chesterfield in November, while there were also spots for Yengi, Gavin Whyte, Lee Evans and Zak Swanson.
Callum Lang, Paddy Lane, Bishop, Owen Moxon and Joe Rafferty were the men to make way, all dropping to the bench.
The sides emerged to a raucous south coast atmosphere, but the home side looked sloppy in the opening stages.
And they were quickly punished, as the Reds worked the ball from left to right and Barry Cotter delivered a cross that was converted by a stretching Cole.
Pompey’s response was instant and when the ball ricocheted into the path of Whyte, his effort was blocked.
But it fell to the feet of YENGI, who turned sharply and slammed a shot into the net to draw them level.
It might have got even better just a few moments later, but a curling Anjorin attempt flew inches past the post.
Things soon got tense again, though, and the Blues were struggling to find any kind of attacking rhythm.
They might have got caught out on the break, with Will Norris tearing from his line to brilliantly deny McAtee.
Mousinho was soon forced into a change, as Swanson went down injured and was replaced by Rafferty.
And despite being well below their best, Pompey saw out the remainder of the half, giving them a chance to regroup at the break.
Half Time: Pompey 1 Barnsley 1
Another switch was made by the hosts during the interval, as Lang replaced Anjorin in the hole just behind Yengi.
They soon had a chance when Evans whipped in a corner from the right, but Sean Raggett could not keep his header down.
And Yengi soon showed a couple of neat touches to work his way into a shooting position before firing inches past the post.
The action was quickly switching from one end of the pitch to the other and neither side was able to take control.
Marlon Pack did well to bludgeon his way forward just before the hour mark, though, and found Whyte, whose shot was blocked.
But Barnsley soon restored their advantage, as Nicky Cadden surged in from the left and threaded a pass through for McAtee to slot home.
Pompey had already been preparing a double switch, as Yengi and Whyte made way for Bishop and Lane.
They might have levelled when a monster Pack throw was flicked on by both Raggett and Connor Ogilvie, with the ball not quite falling for either Lang or Bishop.
Bishop then collected Lane’s pass and smashed a shot against the inside of the post, although the flag had already been raised for offside.
A lofted Evans ball started the next move and it ended with the midfielder hitting a rising effort narrowly over.
Mousinho’s last throw of the dice was an attacking one on 79 minutes, with Christian Saydee on for Evans.
And the newcome immediately made an impact, drawing a clumsy challenge in the box from McAtee that led referee Paul Howard to point to the spot.
BISHOP stepped up to take the penalty and gave Liam Roberts no chance with a fierce strike that flew into the net.
That would have been enough to seal promotion, but the Blues were still looking for the goal that would secure silverware.
And it arrived late on, when Pack swung in a corner that was headed home by SHAUGHNESSY to spark scenes of wild delight.
There was still eight minutes of stoppage-time to see out, but the Blues did just that and can start thinking about a return to Championship football for the first time since 2012.
Pompey (4-2-3-1): Norris; Swanson (Rafferty 31), Shaughnessy, Raggett, Ogilvie; Evans (Saydee 79), Pack (c); Kamara, Anjorin (Lang 46), Whyte (Lane 61); Yengi (Bishop 61)
Goals: Yengi 9, Bishop 83 (pen), Shaughnessy 89
Booked: Evans
Subs not used: Macey, Moxon
Barnsley (3-1-4-1-1): Roberts; Williams, de Gevigney, Earl; Kane; Cotter (O’Keeffe 75), Phillips, Grant (McCart 87), Cadden; McAtee (Jaló 87); Cole
Goals: Cole 6, McAtee 59
Booked: de Gevigney
Subs not used: Killip, Chapman, Marsh, Cosgrove
Referee: Paul Howard
Attendance: 19,960 (309 away fans)
