Flacco, who was released by the Denver Broncos in March after one season, will receive $1.5 million in base pay, plus the chance to make another $3 million with incentives.
The question with Flacco, 35, is his health. The former longtime Baltimore Ravens starter underwent neck surgery in early April to repair a herniated disk and won't be cleared for contact until late August or early September, a source said. He likely will participate in training camp on a limited basis, meaning no contact.
Assuming he's healthy, Flacco will be the primary backup to Sam Darnold.
The Jets had been exploring the backup quarterback market for several weeks. Most notably, they held talks with former Cincinnati Bengals starter Andy Dalton, who wound up signing with the Dallas Cowboys.
The Jets did re-sign journeyman David Fales, but they wanted more insurance at the position. Fales has no career starts and only 48 career pass attempts since being drafted in 2014.
They also used a fourth-round draft pick on James Morgan out of Florida International, but he's projected as the No. 3 quarterback. Fales and former practice-squad player Mike White figure to be the odd men out.
The Jets have struggled with their backups in recent years. Since 2018, they're 0-6 when Darnold didn't start. All told, they've lost 11 straight games with their backup in a starting role. Their last backup to win was Bryce Petty on Dec. 11, 2016, at the San Francisco 49ers.
General manager Joe Douglas is familiar with Flacco, as Douglas was a Ravens area scout when they drafted Flacco in 2008 in the first round. Douglas put a "big-time grade" on Flacco and lobbied hard for him, former Ravens GM Ozzie Newsome said recently in an interview with ESPN.
This is a homecoming of sorts for Flacco, who grew up in Audubon, New Jersey. This will be a new role for him; he has started every game in his career (171).
Flacco was placed on injured reserve halfway through the 2019 season because of the herniated disk. 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 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 a hit on the final play that was one of the biggest he sustained all season. He went on injured reserve five days later.
At the end of last season, Flacco said he hoped to continue playing once he was medically cleared. In 12 NFL seasons, he is 98-73 as a starter with 40,067 passing yards, 218 touchdowns and 141 interceptions.
During four games in the 2012 postseason, Flacco threw 11 touchdown passes without an interception as the Ravens went on to win Super Bowl XLVII.
ESPN's Jeff Legwold contributed to this report.