Brentford Manager Uwe Rosler told the club website: "We knew about Kadeem from when he was at Wycombe and he has developed during his time at Cardiff.
"He offers us something that I have been looking for since we lost Conor McAleny to injury at the end of August.
"We need pace on the flanks. He will give us pace, directness, dribbling ability and will terrorise defences.
"That is what we need to give us another option for our attacking game."
Harris goes into Brentford's squad for today's game with Colchester.