Aimar Govea's 94th-minute strike ensured Swansea City Under-21s made it back-to-back Premier League Cup wins with victory over their West Bromwich Albion counterparts at The Hawthorns.
Swansea led with 10 minutes played after Govea's cross was turned into the back of the net by a West Brom defender.
And the visitors retained their lead at the break with Richard Faakye and Iestyn Jones to the fore defensively.
However, the hosts levelled through Oliver Bostock in the final quarter of the contest.
But Govea netted a dramatic last-gasp winner to claim all three points under the lights.
It was reward for a determined effort from the visitors as they gained more valuable experience playing in a Championship stadium.
The Swans had got off to an impressive start, passing the ball round smartly in tight areas under pressure.
The hosts then had a good spell, and Faakye slid in to cut out one Baggies attack, before Sebastian Dabrowski made an important clearance.
Swansea keeper Ben Hughes was forced in to action when Harry Whitwell won the ball in midfield, eventually parrying away a low strike on goal.
But the Swans took the lead in the 10th minute through an own goal. Govea's high cross from the left ricocheted off the post before deflecting off the helpless Muhamed Diomande and into his own net.
From the restart, West Brom immediately pressed forward through Eseosa Sule, but Dabrowski was on hand to fend off the forward and win a free-kick.
Down the other end, the visitors had two chances from corners, with skipper Mitchell Bates shooting wide before Jones headed the ball off target.
The home side stepped up the pressure with Sule and Cole Deeming firing thunderous efforts over the crossbar.
The closing stages of the first period proved nervy for the Swans, but they dug in to remain ahead at the half-time whistle.
The second half started with West Brom winning a series of set-pieces, but Hughes punched away the final delivery in the sequence to lift the pressure.
Swansea hit back and Dan Watts looked set to tee up Sammy Henia-Kamau for a golden chance, only for the final pass to just have too much on it.
At the other end Hughes was alert to deny Dan Chimeziri from close range after a free-kick had fallen for the West Brom man just outside the six-yard box.
But the Baggies did level when a fine solo run from Bostock ended with him finding the bottom corner.
The game looked set to finish all-square as it headed into four minutes of stoppage time. But the Swans had other ideas.
The ball broke kindly for Govea and, after seeing his first attempt come back off the post, the Ecuador youth international was on hand to slot the rebound into an unguarded net.
The Swans remain top of Group C following their 6-0 victory over Fleetwood Town in September. They welcome Southampton to the Joma High Performance Centre in their next Premier League Cup fixture on Friday, November 8 (7pm).
West Bromwich Albion U21s: Ben Cisse, Deago Nelson, Gianluca Frabotta, Dauda Iddrisa, Muhamed Diomande (captain), Josh Shaw, Fenton Heard, Cole Deeming, Eseosa Sule, Harry Whitwell, Oliver Bostock.
Substitutes: Matt Richards, Joe Wallis, Akeel Higgins, Michael Parker, Dan Chimeziri.
Swansea City Under-21s: Ben Hughes, Arthur Parker, Sebastian Dabrowski, Mitchell Bates (captain), Richard Faakye, Iestyn Jones, Aimar Govea, Dan Watts (Thomas Woodward 77), Sammy Henia-Kamau (Morgan Bates 87), Ben Lloyd, Glory Nzingo.
Substitutes: Josh Pescatore, Kit Margetson, Brogan Popham.