Flacco, who was placed on injured reserve halfway through last season with a herniated disk in his neck, becomes an unrestricted free agent.
He was 2-6 as Denver's starting quarterback in 2019, throwing for 1,822 yards with six touchdowns and five interceptions. He completed 65.3% of his passes.
Flacco, 35, had two years remaining on his contract -- the Broncos renegotiated it in August to give themselves additional salary-cap room -- with cap charges of $23.65 million in 2020 and $27.65 million in 2021.
By releasing Flacco, the Broncos save just over $10 million against the cap.
It is a long way from a year ago, when Flacco arrived in Denver. John Elway, the Broncos' president of football operations and general manager, declared then that Flacco had a "lot of football left in him" and was "just really coming into his prime."
The Broncos struggled mightily on offense last season, but especially in Flacco's starts, as the team scored more than 16 points in just two of his eight games. Flacco took a pounding, getting sacked 26 times.
Flacco said during the season that he had felt discomfort in his neck "for a couple of weeks" before his last start, Oct. 27 in Indianapolis. He took several big hits in the loss to the Colts, including one of the biggest all season on the game's final play.
He went on injured reserve five days later.
Flacco has received two medical opinions on his neck since the end of the season, including an MRI in February. Because of nationwide travel restrictions due to the coronavirus outbreak, Flacco has not been examined by the Broncos' medical staff since his exit physical Dec. 30, but the belief is that Flacco would be medically ready to play in the upcoming season if he is signed.
At the end of the 2019 season, Flacco said he hoped to continue playing once he was medically cleared.
"I'm probably a little bit more worried about other things at this point, but if [being a backup is] what it has to be, I want to play football," Flacco said then. "If that's what it has to be, whether it's here or wherever, if that's what it has to be for me to get back in and start playing again, then yeah, I'll go that route."
Lock, a second-round pick last April, started the Broncos' final five games, and the team went 4-1 down the stretch. Elway said at season's end and at the scouting combine last month that it was "unrealistic" to say Lock would not be the starter moving forward.
In 12 NFL seasons, Flacco is 98-73 as a starter with 40,067 passing yards, 218 touchdowns and 141 interceptions.
During the 2012 postseason, Flacco threw 11 touchdown passes without an interception in four games as the Baltimore Ravens went on to win Super Bowl XLVII.