Hasselbeck will turn 36 in September. His prior contract was signed before the start of the 2005 season and was followed by him leading the Seahawks to their only Super Bowl appearance.
According to the sources, Jackson's deal is for $8 million over two years, an indication he was brought in to compete for the starting job. His contract matches that of current Seahawks quarterback Charlie Whitehurst, who signed a two-year, $8 million contract after being acquired in a trade before last season.
Jackson, Minnesota's second-round pick in 2006, would appear to have an edge in the competition because he has played for current Seahawks offensive coordinator Darrell Bevell when they were both a part of the Vikings.
Meanwhile, the Seahawks are also interested in adding Matt Leinart to compete with Jackson and Whitehurst.
Leinart could reunite with Seahawks coach Pete Carroll, his college coach at USC. Leinart's career has hit a low point after the Arizona Cardinals drafted him in the first round in 2006. The Cardinals released Leinart during the preseason last August and he spent 2010 as a backup with the Houston Texans.
Sources told ESPN senior NFL analyst Chris Mortensen that Leinart was gauging other options Tuesday that include the Washington Redskins. Redskins coach Mike Shanahan graded the left-handed Leinart highly before the 2006 draft when Shanahan was the Broncos' coach.
Whitehurst was one of the first Seahawks to arrive at their training facility on Tuesday morning after catching a late flight back to the Northwest and not getting to his apartment until about 1 a.m. Still, he was driving through the gates of Seahawks headquarters shortly after 8 a.m. ready to pick up his playbook.
"You're more experienced; it goes for all of us. There were a bunch of new guys this past season," Whitehurst said. "I think everybody is probably feeling pretty confident about this season."
Instead of Whitehurst battling Hasselbeck for the starting job, it'll be Jackson. Later in the day, after learning the reports of Hasselbeck's departure, Whitehurst seemed to realize his chance.
"It's as good an opportunity as I'm going to get and I'm looking forward to it," he said.
Jackson's arrival ends an era in Seattle.
Hasselbeck, a three-time Pro Bowl player, was drafted by the Green Bay Packers in the sixth round of the 1998 draft and was traded to the Seahawks in 2001.
The decision to move on from Hasselbeck contradicted Carroll's pronouncement in January that re-signing Hasselbeck was Seattle's No. 1 priority. The two sides could not reach an agreement before the NFL lockout.
The Seahawks had offered Hasselbeck a one year, $7 million contract earlier in the offseason, the sources said, but it was not known if that offer was still on the table this week.
Hasselbeck's first season with Carroll was punctuated by a series of ups and downs, getting booed off the field in his last start of the regular season, countered by the highs of Seattle's 41-36 upset of New Orleans to open the NFC playoffs in one of Hasselbeck's finest performances.
Hasselbeck threw for 272 yards and a career playoff-high four touchdowns in engineering the Seahawks' upset victory over the defending Super Bowl champion Saints. He threw TD passes of 7 and 11 yards to tight end John Carlson, 45 yards to Brandon Stokley and 38 yards to Mike Williams.
It was a stark contrast to his previous start at home in Week 15 against Atlanta, when Hasselbeck was booed off the field and eventually replaced in the third quarter by Whitehurst who also started the season finale against St. Louis and led Seattle to a 16-6 victory that clinched the NFC West title and a playoff spot.
Hasselbeck had been instrumental in organizing offseason workouts for players in the Seattle area and working with the University of Washington on using their facilities. Some players had been extremely outspoken on Twitter in urging the Seahawks to re-sign the veteran QB.
"Gotta say thanks for giving me a chance to get my career rolling," Williams wrote to Hasselbeck on Twitter Tuesday afternoon. "U'll always Rep the 12th, see u in the Ring of Honor sir."
Rich Moran, the agent for Seahawks offensive lineman Stacy Andrews, confirmed Tuesday his client has been informed he will be released. Andrews was scheduled to make $5.25 million this season, but was likely to be a backup after Seattle drafted linemen James Carpenter and John Moffitt.
The Seahawks also agreed to terms with 19 undrafted free agents on Tuesday. Highlighting the list was Boise State safety Jeron Johnson, Nebraska safety Rickey Thenarse and Nebraska defensive end Pierre Allen.
Information from ESPN senior NFL analyst Chris Mortensen, ESPN NFL Insider Adam Schefter, ESPN.com senior NFL writer John Clayton, ESPN.com's Mike Sando and The Associated Press was used in this report.