The Brazilian signed for Barca in 2014 from Sao Paulo, but made just eight appearances in two seasons before being farmed out to Sporting Gijon last season.
And he is set to spend another year away from Camp Nou as the Catalans revealed they had agreed terms with Benfica.
"Barcelona and Benfica have come to an agreement for the loan of Douglas Pereira, 27, who will play in the Portuguese club during the 2017-18 season," Barca explained in a statement.
[ÚLTIMA HORA] Cesión de Douglas al Benfica https://t.co/AVHslnHvy0— FC Barcelona (@FCBarcelona_es) 31 de agosto de 2017
"Douglas goes on loan to Benfica three years after his Barcelona arrival. He came from Sao Paulo in the summer of 2014 and made his debut in September of that year against Malaga.
"During the first two seasons, 2014-15 and 2015-16, Douglas played eight games, with four draws and four victories, and and a total of eight titles."
Douglas becomes the seventh player to leave Camp Nou this season, following compatriots Neymar and Marlon, a fellow loanee who will play at Nice, as well as Sergi Samper, Jordi Masip, Cristian Tello and Jeremy Mathieu in departing on a permanent or temporary basis.