Dan Smith and Bradley Lethbridge (pictured) struck in the final few minutes as Pompey Reserves battled from behind to beat Barnsley in the Premier League Cup.
The youthful Blues side secured their first group stage win at Oakwell to keep alive hopes of reaching the knockout stages.
After a cagey opening, the hosts were reduced to 10 men midway through the first half, with Matty Pearson dismissed for a late lunge on Leon Maloney.
But they broke the deadlock before the interval when Romal Palmer’s free-kick deflected off the wall and flew into the net.
Pompey had a chance to level on the hour mark, but a rising Matt Mayes could not quite guide Jack Chandler’s cross on target.
It was the Reds who looked more likely to add to the scoring, however, and Alex Bass had to beat away Louis Wardle’s shot.
The keeper then made fine saves to deny Jacob Brown and Victor Adeboyejo, while the former also saw an effort strike the crossbar.
But Pompey were level on 88 minutes when substitute Lethbridge whipped in a fine cross from the right and SMITH rose at the far post to convert.
It looked like the game would finish all square, but LETHBRIDGE lashed home a long-range strike in stoppage-time to seal victory for the Blues.
Academy manager Mark Kelly said: “The red card did change the fabric of the game a bit and then Barnsley scored just as we were starting to take a foothold.
"It was a very young side we fielded and so it was a particularly pleasing performance."
“We changed shape after the break because they were sitting back a bit more and playing with just the one man up top.
“There was a wobbly period for us, but Bass made some fantastic saves to keep us in it. He was fantastic.
“We had to stay patient and try to use the wide areas a bit more and eventually that hard work paid off.
“The two goals were both finished well and it was pleasing to see Bradley provide the assist for one before scoring the other.
“It was a very young side we fielded and so it was a particularly pleasing performance. There were plenty of positive aspects and also some things for us to work on.”
Barnsley: Walton; Smith, Pearson, Fielding, Williams; Wardle, Palmer, Wolfe (Tingle 69), Berkovits; Adeboyejo, Brown
Subs not used: Greatorex, Lund, Dolan, Rowe
Pompey: Bass; Oxlade-Chamberlain, Casey, Dandy, Hancott; Chandler (Lethbridge 61), Brooks (Read 57), Widdrington (c), Maloney (Johnston 77); Smith, Mayes
Subs not used: Collins, Whiting