Chesterfield are now just three points shy of a League Two play-off spot after two second-half goals at Underhill.
Barnet started the second half brightly but it was Jay O'Shea who swivelled on Drew Talbot's pass to curl the Spireites into a 57th-minute lead.
Curtis Weston should have equalised when clean through but delayed and crossed when he should have shot.
Tendayi Darikwa's driven cross was then poked home by Sam Togwell, leaving the Bees two points clear of the drop zone.
VIEW FROM THE DRESSING ROOM
Barnet boss Edgar Davids told BBC London 94.9:
"We got a lesson in effective finishing. If you have so many corners and you don't convert one, then you can play for two days and you won't score.
"We were the team that had all the play in the first half and gave few chances, and they scored from a half chance. They showed how to convert.
"If you have so many corners and the better of the play, then you can get something but, if you are not sharp in the box and you don't have the determination to score, then you won't get anything."
Chesterfield manager Paul Cook told BBC Radio Sheffield:
"We played well second half. We looked a lot better in possession. First half we were poor.
"We had some words at half-time. It looked like an end of season game for us, like we had nothing to play for.
"Irrespective of other teams, we can't hand other teams the play-offs on a plate. We must make them work for it. Second half, I thought we did that."