The 27-year-old moved to Spain from Sao Paulo in 2014, but has only played eight times for Barcelona's first team.
The Brazilian returned to Camp Nou this summer after a loan spell at Sporting Gijon last season, but he has not been involved in the opening weeks of the 2017-18 campaign, and has now been allowed to leave on another loan spell.
"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," read a statement from Barcelona.
"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."
A further statement from Barcelona revealed that the deal does not include an option to make the move permanent.