Freeman's agent, Erik Burkhardt, made a one-word announcement on Twitter before confirming his client's decision to The Associated Press.
— Erik Burkhardt (@ErikBurkhardt) October 7, 2013
The Vikings gave Freeman a one-year deal worth about $3 million, a league source told ESPN NFL Insider Adam Schefter.
All totaled, Freeman will make more than $11.4 million until he becomes a free agent this winter, factoring in the $2.479 million the Bucs paid him at the start of the season, the $5.9 million in termination pay from the team, and his new deal with Minnesota.
Freeman also had been linked to the Bills and Raiders since being released by the Bucs on Thursday after a tumultuous season for the quarterback and the team that made him the 17th overall pick in the 2009 draft.
The Vikings still will list Ponder as their starter, a team source told ESPN Senior NFL Analyst Chris Mortensen, though he is recovering from a fractured rib.
"I think Josh Freeman is a good player," a Vikings source told ESPN's Josina Anderson. "We just have to figure out where things went wrong in Tampa Bay so we can help him. He is too young not to progress."
Cassel started last week in a 34-27 win against the Pittsburgh Steelers in London. The team is coming off a bye week as it begins preparations for its Week 6 home game against the Carolina Panthers. With the addition of Freeman, the Vikings are likely to release third-string QB McLeod Bethel-Thompson, sources said.
Buccaneers general manager Mark Dominik contacted all 31 other teams in an attempt to trade Freeman, sources told ESPN.com on Thursday, but were unable to find a taker.
Freeman was in the final year of his rookie contract. The Buccaneers entered the season fully willing to pay him as a franchise quarterback as long as he demonstrated he was worthy with his performance on the field, sources said.
Freeman was benched in favor of rookie Mike Glennon prior to Tampa Bay's 13-10 loss to Arizona in Week 4. In 60 career games, Freeman has thrown for 13,534 yards, 80 touchdowns and 66 interceptions. He is the franchise's leader in career TD passes.
Last season, Freeman threw for 4,065 yards and 27 touchdowns. He had two touchdown passes and three interceptions this season before he was benched.
Information from ESPN's Ed Werder and Chris Mortensen, and ESPN.com's Mike Rodak contributed to this report.