The 20-year-old from Portugal, who has yet to make a first-team appearance at Anfield, will link up with the Bees until January 5.
Brentford manager Uwe Rosler told the club website: "We have been following Joao for a while.
"We have not scored many goals so far this season and it is not down to the lack of opportunities. I feel we needed another addition to give us extra quality in the final third.
"We have watched him and now he has become available we have been able to take him.
"We thank Liverpool Football Club for allowing Joao to come here, he will give us something creative we need, especially when teams sit deep against us."
Earlier on Tuesday Rosler also brought in Sheffield Wednesday defender Martin Taylor on a month's loan.