Keirrison only signed for Barca last week from Palmeiras, but the Treble champs do not have room for him for the coming campaign.
"It has all happened very quickly," explained Keirrison, "but I am really happy. Benfica is a great club, that the whole world knows.
"In Portugal I will play so to adjust to European football. There they speak my language and also in the team there will be other Brazilians."