The Bees controlled the game for long periods after going ahead through Alex Pritchard’s brilliant strike – but missed chances in the second half came back to haunt them when Bent grabbed his 10th goal for the Rams.
Derby went into the game knowing they needed to win to keep their slim hopes of automatic promotion alive and Tom Ince twice went close in the opening seven minutes.
The on-loan Hull winger was caught by Toumani Diagouraga in the fifth minute and curled the 25-yard free-kick over the wall and just past David Button’s left post with the goalkeeper desperately scrambling to get across.
Button had to plunge to his right two minutes later when Ince curled a shot from the edge of the box but Brentford hit back in the 14th minute when Jota’s header set up Moses Odubajo for a shot that Lee Grant pushed away.
Grant was relieved to see a free-kick from Alan Judge whistle past his left post after Pritchard was tripped by Raul Albentosa just outside the area in the 22nd minute – but he was beaten by a quality goal six minutes later.
Diagouraga tackled Jeff Hendrick deep in his own half and found Jonathan Douglas who quickly released Judge, before Pritchard went past Richard Keogh and Craig Bryson and bent a right foot shot from the edge of the box past the despairing dive of Grant.
Pritchard again found space just outside the area but this time he shot straight at Grant before Ince ran down the left of Brentford’s defence to drive in a low shot which Button saved diving to his left.
Brentford were good value for their half-time lead and they came close to increasing it in the 51st minute when Bryson’s attempted clearance spun back to Andre Gray – but Albentosa deflected his shot wide.
Gray had another chance four minutes later when Douglas played him in but fired wide from eight yards, then the home fans found their voice when sub Simon Dawkins played Bent in but Button raced off his line to make the save.
There was a big let-off for Derby in the 68th minute when Jota pulled the ball back from the edge of the six-yard box to tee up Douglas – but his shot was cleared off the line by Craig Forsyth.
Brentford had another chance in the 73rd minute when Stuart Dallas crossed to the back post, where Gray headed over at full stretch and the Rams nearly made them pay when Zak Whitbread flicked on a corner and Button made an instinctive save to push the ball behind.
But Derby grabbed a goal they barely deserved in the second minute of stoppage time when Hendrick fired in a low shot and Bent pounced to stab home an equaliser that was tough on Brentford.