Harris has made just one senior appearance for the Bluebirds since his arrival from Wycombe Wanderers in January, and Brentford manager Uwe Rosler believes that the 22-year-old will become a valuable asset to the squad.
The German told the club's official website: "He offers us something that I have been looking for since we lost Conor McAleny to injury."
"We need pace on the flanks. He will give us pace, directness, dribbling ability and he will terrorise defences.
"That is what we need to give us another option for our attacking game."
Harris is likely to feature in Brentford's League One fixture with Colchester United on Saturday.