Barnsley defied the form book and ended Brentford’s hopes of making the play-offs.
Kieffer Moore scored the opener after 11 minutes before Oliver McBurnie headed in the second six minutes after the break.
The home side had lost more points from winning positions at home than any other team before the game, but they held on to end their run of four games without a win.
Barnsley took a shock early lead through a swift counter-attack.
Gary Gardner won the ball inside his own box, broke to the halfway line and released Moore down the left-hand side. The striker then looked to cross with his weaker left foot but instead saw the ball sail over David Bentley and go in off the post.
Moore should have doubled his tally in the 25th minute when Connor Mahoney sent a delightful cross into the box but the forward could not quite direct his diving header on target and it flashed just wide.
The visitors were beginning to stamp their authority on the game and Oliver Watkins found himself with space to run into and only goalkeeper Jack Walton to beat, however Liam Lindsay somehow recovered and ended the danger with a superb sliding tackle.
Shortly before the break, the Scottish defender made another important block from Sergi Canos’ shot, while – from the resulting corner – Chris Mepham was unlucky to miss with a snap-shot from the edge of the box.
With the Bees failing to take advantage of their pressure, Barnsley struck again six minutes after the restart. Adam Hammill registered his first assist of the season at a pivotal time as his deep-corner delivery found McBurnie, who netted his ninth goal for the club in front of the home fans.
Unbelievably, Moore then missed an open goal from six yards. Mahoney had done well to beat two defenders and cut in on his left, Bentley matched the 21-year-old’s shot but he could only push it to Moore who skied his effort.
Barnsley were full of confidence and Hammill flicked the ball to Moore, but could not quite find the bottom corner with his left foot as he sent it wide again.
Yoann Barbet dragged an effort well wide before Mepham tried to blast an effort in from distance but Brentford could not get past the home defence.