San Francisco had two second-round picks after making a draft-day trade with the New Orleans Saints this year, but the one traded for Garoppolo is its original second-round pick, a source told ESPN's Adam Schefter.
"Albeit in limited game action, Jimmy has displayed the characteristics and traits that we believe are vital to being a successful quarterback in this league. He has had the rare opportunity to sit and learn from a future Hall of Fame quarterback in a championship atmosphere."
49ers GM John Lynch
The 49ers said Garoppolo still has to pass a physical and that a news conference is scheduled for Tuesday afternoon.
The 49ers have also decided to release veteran quarterback Brian Hoyer, a source told Schefter. Hoyer's release has not officially been announced. Once released by the Niners, Hoyer is expected to sign with the Patriots, barring an unexpected turn, a source told ESPN's Mike Reiss.
"We are thrilled for Jimmy to join the 49ers family," 49ers general manager John Lynch said in a statement Tuesday. "Jimmy is a player we have researched extensively since [49ers coach] Kyle [Shanahan] and I joined the 49ers. I am extremely pleased this came together today and we all think it is a big win for our organization.
"Albeit in limited game action, Jimmy has displayed the characteristics and traits that we believe are vital to being a successful quarterback in this league. He has had the rare opportunity to sit and learn from a future Hall of Fame quarterback in a championship atmosphere. We look forward to Jimmy joining the team tomorrow and hitting the ground running."
The 49ers and Patriots began discussing the trade Monday morning, and by evening the trade was officially agreed upon.
Garoppolo, 25, is scheduled to become an unrestricted free agent after the season. The 49ers are likely to keep him by signing him to a new deal or tagging him as their franchise player.
The Patriots would seem to have bought in to the idea that Tom Brady can play well into his 40s and are left without a backup at quarterback, having traded Jacoby Brissett to the Indianapolis Colts in September for receiver Phillip Dorsett.
The Patriots are on their bye week, during which they will have extra time to find a backup via trade or free agency. They will have until Tuesday's 4 p.m. ET trade deadline to explore that avenue.
Brady, speaking in his weekly Monday night Westwood One radio interview, said he wasn't sure of New England's plans.
"I really don't know," Brady said. "I've been at home all day and learned the news at home. That's up for everyone else to decide. My job is quarterback, and that's what I'm focused on."
Brady said he hoped Garoppolo will make a good starting quarterback for the 49ers.
"He's put in all the effort," Brady said. "I've watched him for three and a half years and really enjoyed working with him. Hopefully he goes on and does a great job."
Garoppolo started New England's first two games in 2016 amid Brady's four-game Deflategate suspension before spraining his shoulder. Garoppolo was 42-of-59 passing with 4 touchdowns and 0 interceptions in the two wins for the eventual Super Bowl champions. For Garoppolo, a second-round pick in the 2014 draft by the Patriots out of Eastern Illinois, it has been his only significant playing time in the NFL.
The 49ers chose not to seek a potential long-term quarterback solution in the offseason, instead signing Hoyer and using a third-round pick on Beathard.
Hoyer started the first six games but was benched in favor of Beathard in the team's loss to Washington on Oct. 16. Beathard has started the past two games but didn't fare much better than Hoyer as the Niners lost to Dallas and Philadelphia by a combined 53 points to drop to 0-8 for the first time in franchise history.
The trade for Garoppolo comes as a surprise if only because most Niners quarterback speculation centered on Cousins, who could become a free agent in March and has long-standing ties to San Francisco coach Kyle Shanahan.
In trading for Garoppolo now, the 49ers have the chance to get him into Shanahan's system and control his rights with the franchise or transition tag should they be unable to sign him to a long-term contract extension.
ESPN's Mike Reiss and Nick Wagoner contributed to this report.