Jackson, 34, was released Monday by theBuffalo Bills. He averaged a career-low 3.7 yards per carry last season, but his 66 catches were a career high.
Seahawks coach Pete Carroll was asked earlier this week what he liked about Jackson, who had scheduled a visit with the team.
"He's been a very good pass-catcher and [an] adept pass protector," Carroll said. "He's always been a good runner. He's got great feel and sense. Had a big run in the game last week (a preseason win against the Pittsburgh Steelers). Had a 40-yard run in the game last week (against the Steelers) and scored a touchdown. But he's been a very adept pass-receiver, and I think that's something that maybe he could fit in."
According to Pro Football Focus, Seahawks starter Marshawn Lynch played the sixth-most snaps of any running back in the NFL in 2014. Lynch, 29, has had 280 carries or more in four straight seasons. Jackson will probably be used to spell Lynch in obvious passing situations.
The two running backs have had a close relationship since 2007, when both broke into the NFL with the Bills. Lynch was Buffalo's first-round pick, and Jackson made the Bills' roster after spending the previous year on their practice squad.
But Carroll said that wasn't why the Seahawks had brought in Jackson to visit.
"Only in that I know that Marshawn and he are very good friends, and they get along, and that's always a good thing," Carroll said. "But other than that, that didn't feed into this."
Adding Jackson became more urgent after Robert Turbin suffered what Carroll termed a "significant" high ankle sprain in the Seahawks' third preseason game. The Seahawks will waive Turbin, who has never missed a game in his three-year career, with an injury designation, according to multiple reports. He is expected to miss about five weeks.
In addition to Lynch and Jackson, the team probably will go into Week 1 with Christine Michael as a third option in the backfield.
Jackson signs one-year deal with Seahawks
ESPN Seahawks reporter Sheil Kapadia explains why the Seahawks made the move to sign 34-year-old Fred Jackson.