Pereira swaps United for Flamengo, who have the option to buy the one-time Brazil international at the end of the loan deal.
It comes after Flamengo signed Kenedy on loan from Premier League powerhouse and Champions League holders Chelsea.
"Amazing feeling," Pereira told Flamengo TV after arriving at the airport. "I am very happy. I want to work, I want to get here to win titles, help the team and do my best.
"It's an inexplicable feeling. I did the entire pre-season with Manchester. So I'm prepared to play. I'm really looking forward to starting work."
Pereira has made 75 appearances and scored four goals across all competitions since his United debut in 2015, however, the 25-year-old will spend another season away from Old Trafford.
After loan spells at Granada in 2016-17 and Valencia in 2017-18, Pereira was loaned to Serie A's Lazio last season, scoring one goal in 33 appearances.