Rochdale slipped out of the League One play-off places after Robbie Simpson grabbed a late winner for Brentford.
The home side dominated but wasted their chances with Will Atkinson firing wide and Gary Jones going close with a 25 yard strike.
Dale had been harbouring hopes of a second successive promotion after winning six of their last eight games.
Simpson secured the late win for the visitors though, heading in Sam Saunders' cross from the right.
Rochdale manager Keith Hill told BBC Radio Manchester:
"It's been really difficult all week trying to wrestle with what to do, the lads deserve credit for the efforts but it lacked energy.
"It was like flogging a dead horse, we've rinsed every ounce of energy from these players.
"It was a great away performance from Brentford and it was always obvious one goal was going to decide the game."
Brentford boss Nicky Forster told London 94.9:
"I thought we were magnificent today and I can't praise all the team enough.
"I think we started the game very brightly in the first five minutes and perhaps lost the momentum for about ten minutes.
"I thought we dominated the game and we passed the ball very well."