The Ravens will receive a fourth-round pick in return, a source told Albert Breer of The MMQB. The trade can't be made official until the beginning of the new league year on March 13.
Flacco, 34, is a former Super Bowl MVP, but he's largely underwhelmed since Baltimore's championship season in 2012. Last year, the veteran lost his starting job to rookie quarterback Lamar Jackson, whom the Ravens committed to building around after drafting him in the first round.
In nine appearances during 2018, Flacco threw for 2,465 yards with 12 touchdowns and six interceptions.
The reported trade thrusts the future of incumbent Broncos starter Case Keenum into jeopardy. Keenum signed a two-year deal with Denver last offseason, but struggled throughout his first campaign in the Mile High City.
Flacco has three years remaining on his existing deal, and will earn $18.5 million in base salary in 2019 unless he reworks his contract.
He was a first-round pick of the Ravens in 2008.