Rob Dickie was sent off and Yoann Barbet scored a comical own goal as QPR’s worrying run continued.
Cauley Woodrow netted with a 27th-minute penalty after Dickie was sent off for a foul on the former Fulham striker.
Conor Chaplin’s goal 10 minutes later put Barnsley in complete control.
And Barbet’s gaffe midway through the second half summed up a miserable night for struggling Rangers, who are now without a win in seven matches, have failed to score in their past four and have won just three of their 18 games since the coronavirus lockdown.
They had plenty of early possession at Oakwell but were undone by Alex Mowatt’s defence-splitting pass after Macauley Bonne had given the ball away.
Woodrow nipped between Barbet and Dickie, who pulled him down and was immediately red-carded.
Woodrow emphatically put away the resulting spot-kick and Chaplin doubled Barnsley’s lead by firing past Seny Dieng at the keeper’s near post after being set up by Clarke Odour.
The third goal came after a Barnsley attack seemed to have broken down after Woodrow’s heavy touch.
Barbet, in attempting to lay the ball back to Dieng inside the penalty area, instead prodded it past him and into the net.
QPR: Dieng, Kakay, Dickie, Barbet, Hamalainen, Ball, Carroll, Adomah (Kane 54), Chair (Masterson 31), Bonne (Willock 63), Dykes.
Subs not used: Kelly, Bettache, Cameron, Kelman.