AFC Bournemouth under-18s manager Alan Connell hailed the “abundance of character” from his players to come from three goals down to stun Saints 4-3 at Staplewood.
Saints were two goals up at half-time thanks to strikes from Stefan Vukoje and Sam Bellis.
The hosts then added a third thanks to Vukoje after the break.
But Cherries rallied through Jake Adams’s double before Christian Saydee found a third for Connell’s men to restore parity.
Marcus Daws then struck the winner for Cherries against his former club, to ensure a remarkable second-half comeback.
Connell told the Cherries website: “We started the game very sloppily, both with and without the ball, and in any game of football if you do that you're going to struggle.
“Southampton have a lot of quality and they showed that to go two goals up.
"It probably took us about 25 minutes to get into our flow in the game but once we did that we were more on the front foot which is what I wanted from the start.
"We had a big second half in front of us and the boys showed more intensity, more aggression and if we do that with the quality we've got against any opposition, even category one like Southampton, we can impress against anyone.
"What pleased me even more from a development point of view was that we conceded straight after half-time. To go 3-0 down, then we were really in a difficult situation, what did we have?
"I thought the lads showed an abundance of character and quality to produce an excellent second-half performance and show that resolve to score some excellent team and individual goals."
Both sides were also able to field some under-21 players during the friendly.
Saydee, Brooklyn Genesini, Nathan Moriah-Welsh and Ben Greenwood all played for Cherries against their south coast rivals.
"It was a really good day with lots of things to learn, it wasn't all perfect and there were lots of things that could have been done better, but I'm delighted overall,” said Connell.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here