The visitors played some good football in the early stages and were rewarded when Matthew Hill gave them the lead with 17 minutes gone.
It got better for the Tykes five minutes after the break when Kieran Trippier crossed for former Forest striker Andy Gray to head in.
Forest looked to be dead and buried but Lewis McGugan pulled one back from the penalty spot before substitute Nathan Tyson scrambled home an equaliser 10 minutes from time.
The Reds pushed for a winner late on but they had to settle for a point which kept them in ninth place in the npower Championship, while Barnsley slipped down to 18th.
Forest went into the game on a high after their midweek mauling of rivals Derby and they made a good start.
McGugan cleverly drifted away from two challenges and played the ball into the path of Chris Cohen but his cross was just about cleared with Robert Earnshaw waiting.
However, Barnsley got a foothold in the game and could have taken the lead with 15 minutes gone when Adam Hammill's crossfield ball was volleyed across goal by Trippier to Gray who saw his shot saved superbly by Lee Camp.
Two minutes later the visitors did go in front when Hill robbed the ball from Marcus Tudgay on the halfway line and ran into the area unchallenged before firing past Camp.
The goal stunned the home crowd and Wes Morgan could have drawn Forest level soon after but he headed over from six yards after being found by McGugan's free-kick.
With half-time approaching Radoslaw Majewski linked up well with Earnshaw but Barnsley goalkeeper Luke Steele smothered the Welshman's shot.
Forest boss Billy Davies would have been looking for his side to put Barnsley under pressure early in the second half but the visitors surprisingly doubled their lead five minutes after the break.
Trippier got away down the right and outfoxed Cohen before supplying a pinpoint cross for Gray who made no mistake with a downward header from close range.
The home side had a lifeline with 68 minutes gone when Chris Gunter was brought down by Hammill in the area and McGugan fired the resulting penalty low into the bottom corner.
Forest should have been level two minutes later but Earnshaw scooped substitute Tyson's cutback over the bar.
With 10 minutes remaining Barnsley failed to clear their lines and Tyson pounced on the loose ball to fire in and send the City Ground into raptures.