ESPN.com first reported the deal.
Seattle said it will get a fourth-round pick in exchange for Branch getting a reunion with the team that drafted him in 2002.
"(He's) excited as can be," Branch's agent, Jason Chayut said Monday night.
The trade ended a day of speculation after Branch was absent from Seahawks practice.
Seattle coach Pete Carroll was nearly silent on Branch's status, other than saying he gave Branch an excused "personal day" on Monday as the Seahawks returned from their bye.
Carroll added he expected Branch to be with the team at its next practice on Wednesday.
"At this point I do, yes," Carroll said of Branch being around later in the week.
Obviously, that was not the case. Reports first surfaced last week of talks between the Seahawks and Patriots following the trade of Randy Moss from New England to Minnesota.
The normally talkative Carroll said just 28 words in response to any questions about Branch.
Branch joins a young New England receiving group, sans veteran Wes Welker. Aside from Welker, second-year receiver Brandon Tate is the only other Patriots receiver with more than 10 catches.
Branch is signed through next season. He is schedule to make $5.45 million in base salary this year and $5.95 million in 2011.
Before coming to Seattle, Branch was a rising star with the Patriots. He caught a career-best 78 passes for nearly 1,000 yards and five touchdowns in 2005 and that followed up his Super Bowl MVP performance a year earlier. Branch had 11 catches for 133 yards in the Super Bowl against Philadelphia.
Branch arrived in Seattle in 2006, acquired from the Patriots for a first-round draft pick, but that first season in Seattle was the only time Branch was able to stay healthy. His career in Seattle was mostly defined by injuries, missing parts of the previous three seasons with some sort of ailment.
Most of Branch's ailments revolved around his left knee he first injured in a snowy playoff game against Green Bay in January 2008. He also missed time with hamstring and foot injuries.
So far this season, Branch has remained healthy. He's caught 13 passes for 112 yards and a touchdown, but had just one catch in Seattle's 20-3 loss to St. Louis.
His future with Seattle was brought into question when the Seahawks signed Brandon Stokley. Just a few days after being signed, Stokley led the team in catches and targets in their 20-3 loss.
Branch was in Louisville, Ky., over the weekend where his number was honored at the University of Louisville. He told the Louisville Courier-Journal he was trying not to focus on the rumors.
"I'm going to leave that to the people that handle that," Branch told the paper. "Right now I'm still a Seattle Seahawk, and whatever happens, happens."