Guy Moussi's injury-time strike ended Barnsley's resistance to send the home fans into raptures.
Forest now have 22 points from their 13 games and are establishing themselves as firm play-off contenders, if not candidates for automatic promotion in what is a wide-open Championship.
Over 20,000 were in attendance and Billy Davies will have been delighted that his side followed up Saturday's win over Newcastle with a hard-fought victory in a less glamorous looking fixture.