Craig Davies struck four times in 19 second-half minutes as Barnsley outclassed Birmingham in front of a less than half-full St Andrew's.
It was a first clean sheet in 24 away games for the Tykes, who had lost their last five league matches on the road.
The first goal did not arrive until the 50th minute from Stephen Forster at the far post after two men failed to clear.
Davies scored a hat-trick in 11 minutes as the Blues, later down to 10 men due to injury, were torn open in midfield.
In six of their last nine visits to St Andrew's Barnsley had failed to find the net and in 13 matches on their travels since January they had recorded only one point.
They were the better side throughout an extraordinary match but neither team had an effort on target in the opening half hour.
Barnsley at St Andrew's
Prior to this match Barnsley had won only two league matches in 18 visits to Birmingham since 1950.
Keith Hill's side took the lead after a catalogue of errors from Birmingham that began with a skewed clearance from Jonathan Spector to gift the visitors a corner.
The set piece was completely missed by Marlon King and Forster was able to bundle home from inside the six-yard box.
Another defensive blunder allowed Barnsley to seize possession again and the ball was squared to Davies, who poked in from close range.
To the dismay of the 13,893 crowd, the visitors continued to carve open the Birmingham defence with some neat and simple passing movements, and Davies found himself in space again in the left channel before cutting inside, holding off two defenders and drilling a powerful shot into the bottom corner.
The 26-year-old Welsh international, who joined Barnsley from Chesterfield in 2011, waltzed through for his hat-trick, before Chris Dagnall was left unmarked at a right-wing throw-in to deliver a perfect cross for Davies to head in at the far post for Barnsley's fifth.
It was the Yorkshire side's biggest win since 1999 as Birmingham, who had only one shot on target in the match, succumbed to their worst home defeat for 25 years and manager Lee Clark kept his players locked in the dressing room for some time after the final whistle.
Birmingham manager Lee Clark:
"Losing any game at home is a massive disappointment but to lose by this margin is devastating.
"Any loss is tough to take but this one is very tough.
"We never started the game. We were lucky it was goalless at half-time in all honesty.
"It was a totally flat performance in every aspect. Barnsley played through us. It's an awful performance. There were lots of mistakes but everything we did was so flat. There was no urgency and it was lifeless."
Barnsley boss Keith Hill:
"There has been no better result but we performed equally well at Blackburn in midweek and got beaten.
"I am really pleased for everyone who believes in what we are trying to achieve at Barnsley.
"Craig Davies takes the honours but this performance has been achieved from the groundwork he put in last season.
"You have to keep players to educate them and Davies is a mobile target man who scores goals. It's as simple as that."
- 01 Butland
- 03 Murphy (Ambrose - 25' )
- 04 Caldwell
- 05 Pablo
- 23 Spector
- 07 Burke
- 08 Mullins (Redmond - 55' Booked )
- 15 Elliott
- 18 Fahey
- 09 King
- 29 Lita (Zigic - 61' )
- 13 Doyle
- 06 Davies
- 10 Ambrose
- 12 Morrison
- 22 Redmond
- 11 Lovenkrands
- 19 Zigic
- 12 Alnwick
- 03 Golbourne
- 06 Foster
- 14 Wiseman
- 25 Cranie
- 33 Kennedy
- 07 Dawson Booked
- 19 Cywka
- 21 Mellis
- 09 Davies Booked (Harewood - 85' )
- 13 Dagnall (Rose - 87' )
- 01 Steele
- 10 Done
- 11 Perkins
- 16 Harewood
- 23 Etuhu
- 24 Noble-Lazarus
- 32 Rose
Live Text Commentary
Full Time The referee blows his whistle to end the game.90:00 +3:16
Tomasz Cywka gives away a free kick for an unfair challenge on Wade Elliott. Wade Elliott takes the free kick.90:00 +1:44
Marlon King is ruled offside. Ben Alnwick takes the free kick.90:00 +0:35
Short corner taken by Tom Kennedy from the right by-line.89:20
Foul by Steven Caldwell on Marlon Harewood, free kick awarded. Martin Cranie takes the direct free kick.88:36
Shot from long range by Christopher Burke goes over the bar.88:15
Free kick awarded for a foul by Stephen Foster on Marlon King. Keith Fahey takes the direct free kick.86:39 Substitution
Substitution Danny Rose replaces Chris Dagnall.84:23
Tom Kennedy restarts play with the free kick.84:23 Substitution
Substitution Marlon Harewood on for Craig Davies.84:23
Keith Fahey challenges Stephen Dawson unfairly and gives away a free kick.82:43
Effort by Stephen Dawson from a long way out goes over the crossbar.76:23
Unfair challenge on Jonathan Spector by Chris Dagnall results in a free kick. Jonathan Spector restarts play with the free kick.74:47 Booking
Booking Stephen Dawson receives a yellow card.74:22 Booking
Booking Nathan Redmond shown a yellow card.72:22
Assist by Chris Dagnall.72:22 Goal scored
Goal! - Craig Davies - Birmingham 0 - 5 Barnsley Craig Davies scores a headed goal from close range. Birmingham 0-5 Barnsley.70:56
Steven Caldwell gives away a free kick for an unfair challenge on Craig Davies. Tomasz Cywka takes the free kick.67:58
Marlon King produces a cross.67:35
The referee blows for offside. Free kick taken by Jack Butland.
Assist on the goal came from Jacob Mellis.64:21 Goal scored
Goal! - Craig Davies - Birmingham 0 - 4 Barnsley Goal scored by Craig Davies from deep inside the penalty box low into the middle of the goal. Birmingham 0-4 Barnsley.63:06
Free kick awarded for a foul by Keith Fahey on Chris Dagnall. Jacob Mellis restarts play with the free kick.61:50
Jacob Mellis has an effort at goal from deep inside the area missing to the right of the target.60:58
The referee blows for offside. Ben Alnwick takes the indirect free kick.60:40
The referee blows for offside. Jack Butland takes the free kick.60:27 Substitution
Substitution Nikola Zigic comes on in place of Leroy Lita.59:17
Chris Dagnall provided the assist for the goal.59:17 Goal scored
Goal! - Craig Davies - Birmingham 0 - 3 Barnsley A goal is scored by Craig Davies from inside the penalty area to the bottom right corner of the goal. Birmingham 0-3 Barnsley.57:56
Free kick awarded for a foul by Tomasz Cywka on Pablo Ibanez. Direct free kick taken by Pablo Ibanez.57:05
Chris Dagnall takes a shot. Jack Butland makes a save.55:50
Leroy Lita challenges Scott Golbourne unfairly and gives away a free kick. Ben Alnwick takes the free kick.55:14
Corner taken by Jacob Mellis from the left by-line to the near post, Marlon King makes a clearance.54:14
Jack Butland takes the indirect free kick.54:14 Substitution
Substitution Hayden Mullins goes off and Nathan Redmond comes on.54:14
The official flags Craig Davies offside.53:10 Booking
Booking Craig Davies receives a yellow card for dissent.53:04
The assist for the goal came from Jacob Mellis.53:04 Goal scored
Goal! - Craig Davies - Birmingham 0 - 2 Barnsley Goal scored by Craig Davies from inside the penalty area to the bottom right corner of the goal. Birmingham 0-2 Barnsley.52:00
Free kick awarded for an unfair challenge on Stephen Dawson by Keith Fahey. Direct free kick taken by Stephen Dawson.49:18
Jacob Mellis provided the assist for the goal.49:18 Goal scored
Goal! - Stephen Foster - Birmingham 0 - 1 Barnsley Stephen Foster finds the net with a goal from close in to the bottom right corner of the goal. Birmingham 0-1 Barnsley.48:49
Corner taken by Jacob Mellis from the left by-line.48:08
The assistant referee signals for offside against Marlon King. Ben Alnwick restarts play with the free kick.45:01
The game restarts for the second half.45:00 +1:08 Half time
Half Time It is the end of the first-half.43:28
The assistant referee flags for offside against Marlon King. Ben Alnwick takes the free kick.42:53
Wade Elliott takes a shot. Save made by Ben Alnwick.41:40
The offside flag is raised against Jacob Mellis. Indirect free kick taken by Jack Butland.38:05
The assistant referee signals for offside against Chris Dagnall. Indirect free kick taken by Jack Butland.37:07
Chris Dagnall takes a shot. Jack Butland makes a save.36:11
Craig Davies takes a shot. Steven Caldwell gets a block in.35:00
Tomasz Cywka concedes a free kick for a foul on Darren Ambrose. Darren Ambrose produces a strike on goal direct from the free kick.33:25
Inswinging corner taken by Jacob Mellis, Pablo Ibanez makes a clearance.33:00
Chris Dagnall takes a shot. Save by Jack Butland.30:20
Tomasz Cywka crosses the ball, Christopher Burke manages to make a clearance.28:26
Craig Davies gives away a free kick for an unfair challenge on Darren Ambrose. Direct free kick taken by Steven Caldwell.28:02
Free kick awarded for a foul by Leroy Lita on Scott Golbourne. Ben Alnwick restarts play with the free kick.25:49
Corner taken right-footed by Christopher Burke.23:55 Substitution
Substitution Darren Ambrose replaces David Murphy.23:55
Tomasz Cywka has an effort at goal from outside the box which misses left.22:25
Jacob Mellis takes a shot from long distance which goes wide of the right-hand post.21:41
Scott Golbourne produces a cross.19:20
Keith Fahey delivers the ball, Ben Alnwick makes a save.18:39
Unfair challenge on Leroy Lita by Scott Wiseman results in a free kick. Direct free kick taken by Hayden Mullins.14:37
Chris Dagnall sends in a cross, save by Jack Butland.13:22
Chris Dagnall challenges Jonathan Spector unfairly and gives away a free kick. Free kick taken by Steven Caldwell.12:17
Inswinging corner taken right-footed by Wade Elliott, save by Ben Alnwick.10:55
Free kick awarded for a foul by Craig Davies on Christopher Burke. Free kick taken by Jack Butland.9:53
Inswinging corner taken by Jacob Mellis, Marlon King makes a clearance.9:30
Unfair challenge on Jacob Mellis by Hayden Mullins results in a free kick. Scott Wiseman takes the free kick.6:08
Inswinging corner taken by Jacob Mellis from the left by-line, clearance made by Jonathan Spector.5:57
The ball is sent over by Scott Golbourne, Hayden Mullins manages to make a clearance.4:36
Tomasz Cywka crosses the ball, Jack Butland makes a save.4:23
Unfair challenge on Chris Dagnall by Pablo Ibanez results in a free kick. Free kick taken by Tomasz Cywka.3:44
Craig Davies has an effort at goal from just outside the box which goes wide of the left-hand upright.0:58
Pablo Ibanez fouled by Craig Davies, the ref awards a free kick. Free kick taken by Jack Butland.0:00
The match has kicked off.
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