Andre Green scored the late winner as Pompey deservedly beat Arsenal under-21s 2-1 in the Checkatrade Trophy at Fratton Park.
Brett Pitman had earlier netted in the first half before Arsenal grabbed an equaliser against the run of play after the break.
It was an excellent performance from the Blues on the night and they could easily have had more goals.
Centre-back Matt Casey and midfielder Freddie Read were handed their Pompey debuts as boss Kenny Jackett made 11 changes from Sunday’s 1-0 FA Cup victory over Rochdale.
Adam May returned from his loan spell at Aldershot to start behind Brett Pitman who led the line in attack.
Arsenal got on the front foot early on and Bukayo Saka turned sharply in the box but saw his weak low shot easily saved by Luke McGee.
Pompey threatened when Anton Walkes was sent clear by Matt Casey’s superb pass.
The right-back ran into the box and scooped the ball over the Arsenal keeper, only for the covering Danny Ballard to bundle it off the line.
Pompey took the lead on 10 minutes.
Walkes headed back a corner into the danger area and Pitman spun as he fell to flick the ball into the net from inside the six-yard box.
It was a nice finish from the Blues captain and a deserved goal.
Pompey looked the most likely to score again.
Dennis headed a good chance wide after an excellent cross from Green.
Adam May’s low shot slammed against the post from 25-yards then Louis Dennis sent a dipping effort narrowly over the bar.
Christian Burgess was also denied when his low shot was hooked off the line by Ballard.
Arsenal captain Charlie Gilmour shot over the bar on 56 minutes after a quiet start to the second half.
Then on 60 minutes a superb left-foot shot from May thumped against the crossbar with the visiting keeper Deyan Iliev beaten.
Arsenal delivered a shock equaliser in stunning fashion on 66 minutes.
Saka cut inside unchallenged off the left flank and fired a right-foot shot high into the far corner of the net giving McGee no chance.
On 68 minutes it was nearly 2-1 to the visitors.
Tyreece John-Jules was played in but a great block from Burgess deflected his shot over the top when he looked set to score.
Pompey were denied by the woodwork for the third time on 75 minutes.
Walkes crossed and Pitman rose highest but his header hit the inside of the post, rolled along the line and away to safety.
The chances kept coming for the hosts who went in search of a winner.
Pitman headed wide from a good position after a free-kick from Dennis on 78 minutes.
May put Dennis through and he saw his effort clip the keeper then divert just the wrong side of the far post.
On 79 minutes Green shot low and Iliev saved well.
Pompey were back ahead with a very well-worked goal on 83 minutes.
Pitman’s clever flick set Dennis away on the left side of the box and his low ball to the far post found Green who swept the ball into the open goal.
It was a deserved winner with the home side carving out chance after chance.
Pompey (4-2-3-1): Luke McGee, Anton Walkes, Christian Burgess, Matt Casey, Brandon Haunstrup, Danny Rose, Freddie Read, Adam May, Andre Green, Louis Dennis, Brett Pitman. Subs: Leon Pitman, Oscar Johnston, Josh Flint, Leon Maloney, Haji Mnoga, Dan Smith, Joe Mason.
Arsenal under-21 (3-4-3: Deyan Iliev, Jordi Osei-Tutu, Danny Ballard, Laurent Koscielny, Zech Medley, Cohen Bramall, Tobi Omole, Charlie Gilmour, James Olayinka, Bukayo Saka, Tyreece John-Jules.
Subs: Karl Hein, Tolaji Bola, Joe Olowu, Dominic Thompson, Nathan Tormey, Gedion Zelalem, Trae Coyle.
Attendance: 3,109 (188 away fans)