Flacco, 38, replaces rookie Dorian Thompson-Robinson, who sustained a concussion in last weekend's loss to the Denver Broncos and remains in the protocol. Cleveland ruled out Thompson-Robinson on Friday, clearing the way for Flacco to make the start.
Flacco, the Super Bowl MVP with the champion Baltimore Ravens during the 2012 season, signed with Cleveland on Nov. 20. That move came a week after Watson suffered a fracture to his throwing shoulder.
"Joe's a pro," Browns coach Kevin Stefanski said Friday. "I've been around pros, he's right up there. He does his job. Very, very intelligent."
Flacco said Wednesday that the potential to start again is "definitely exciting," especially after he remained an unsigned free agent during the first half of the season while waiting for an opportunity.
"There's so many things that add to it now, having young kids and feeling their excitement about it," Flacco said. "And when you are away, it puts things in perspective a little bit."
Flacco, who has thrown for 42,320 yards and 232 touchdowns in his career, started four games for the New York Jets in 2022.
His past four touchdown passes came against the Browns in Week 2 last season. To cap New York's improbable 31-30 comeback victory over Cleveland, Flacco threw the game winner to Garrett Wilson with 22 seconds left.
Several Browns players and coaches have praised the way Flacco has looked in practice this week, as he took snaps with the first-team offense for the first time.
"He throws a very pretty ball," wide receiver Amari Cooper said. "I don't know if it's intentional or what, but when he drops back and he lets it go, the whole motion is like poetry in motion. It looks good."
Since 1950, the Browns have started four quarterbacks in a season only twice before: in 2008 (Brady Quinn, Bruce Gradkowski, Derek Anderson, Ken Dorsey) and 1988 (Bernie Kosar, Don Strock, Gary Danielson, Mike Pagel).
Cleveland, however, never has had four quarterbacks win as starters in the same season.
The last NFL team to have four different starting quarterbacks win a game in a single season was the 2015 Houston Texans, according to ESPN Stats & Information.
Also Friday, Browns wide receiver Michael Woods II was suspended for the final six games of the season for violating the NFL's personal conduct policy. Woods, a sixth-round pick last year, tore an Achilles in the offseason and hasn't been around the team all season.