Malta international Daniel Bogdanovic headed Barnsley into an early lead at Oakwell on Tuesday night and fired his 14th goal of the season from the penalty spot midway through the second period.
Forest, who have seen an eight-point gap open up between themselves and second-placed West Brom, ensured a frantic finale after substitute Dexter Blackstock pulled one back from close range with 12 minutes left.
But Billy Davies' side are not the force they were at the turn of the year, and with skipper Paul McKenna among several key players injured there was no way back for the visitors.
Mark Robins' Barnsley, thrashed 6-1 at Newcastle 10 days ago, continue to defy the odds and could even worry the play-off contenders if they can sustain the levels they reached at times tonight.
Barnsley goalkeeper Luke Steele was back from suspension and defender Luke Potter was recalled, while James Perch, Dele Adebola, Rob Earnshaw and Radoslaw Majewski returned for Forest.
The visitors started with real purpose in a bid to halt their shocking recent away form, but it was Barnsley who stole into a 12th-minute lead.
Nathan Doyle's free-kick into the penalty area came off Barnsley captain Stephen Foster and Bogdanovic looped a header over Forest goalkeeper Lee Camp for his fifth goal in six matches.
Camp then turned away winger Adam Hammill's curling effort, while at the other end Adebola failed to hit the target when one-on-one with Steele.
Adebola spurned another clear-cut chance in the 20th minute, sidefooting a bouncing ball wide in almost identical circumstances.
Hammill limped off injured midway through the first period but Iceland international Emil Hallfredsson proved a cause for concern in the Forest ranks.
Forest made few inroads throughout the remainder of the first half and were under pressure early in the second period when Bogdanovic pulled a left-footed shot inches wide of the post.
Forest almost hauled themselves level in the 53rd minute when Majewski's thumping drive hit the upright and Chris Cohen curled a shot from the edge of the area just off target.
But Barnsley were stretching the visitors' back four and it was no real surprise when the home side doubled their lead.
Midfielder Filipe Teixeira wriggled clear on the right edge of the area and when he went down under Earnshaw's ill-timed tackle, Bogdanovic stepped forward to convert the subsequent penalty, low into Camp's bottom left-hand corner in the 65th minute.
Earnshaw was booked for his clumsy lunge and Teixeira soon followed him into Leicestershire referee Kevin Friend's notebook as the pace quickened.
Blackstock replaced Adebola in the 73rd minute, while Bogdanovic received a hearty Oakwell send-off when he made way for Iain Hume.
Steele pushed away Majewski's stinging shot, but after keeping out Blackstock's point-blank header he was helpless as the Forest striker followed up to prod home on the rebound in the 78th minute.
Earnshaw's close-range effort was ruled out by referee Friend in the final minute and Barnsley survived five more minutes of time added on.