Harris, a 2017 first-round pick out of Missouri, totaled 3.5 sacks over 41 games (eight starts) with Miami. The Dolphins attempted to move Harris to a 3-4 outside linebacker role in 2019 after his early struggles as a 4-3 defensive end, but he registered just a half-sack in 14 games (five starts) last season.
Friday's move comes just one day after the Dolphins released defensive end Taco Charlton.
Harris worked out for the Falcons before the 2017 NFL draft. At the time, Harris said Falcons coach Dan Quinn was "great'' and "I like the defense they run."
Atlanta drafted defensive end Takk McKinley at No. 26 overall in 2017, four spots after Miami selected Harris. On Wednesday, the Falcons declined the fifth-year option on McKinley's contract.
Going into the offseason, the Falcons were intent on improving a pass rush that has generated a league-low 23% pressure on dropbacks over the past two seasons. They signed free agent Dante Fowler Jr. from the Los Angeles Rams and drafted versatile defensive lineman Marlon Davidson in the second round after letting former sack champion Vic Beasley Jr. sign with the Tennessee Titans.
Harris and Charlton were both unlikely to make the Dolphins' 2020 roster after a massive front-seven overhaul that included signing Shaq Lawson, Emmanuel Ogbah and Kyle Van Noy and drafting Raekwon Davis, Jason Strowbridge and Curtis Weaver.
The Harris and Charlton moves come just days before the May 4 deadline to make a decision on whether to exercise their fifth-year options. The moves save Miami $3.3 million in salary-cap space.
ESPN's Vaughn McClure contributed to this report.