A source with knowledge of the negotiations told Anderson via text the one-year deal is worth $3 million.
"It was a very tough decision for me because of the relationships that I have in Minnesota," Winfield told Anderson. "I've been there nine years. I've had some really great coaches, teammates and fans there. I'm very grateful to the Vikings organization for my experience.
"I'm definitely excited about the opportunity to play in Seattle. They're a great football team. Those guys did an excellent job last season. They're young, athletic and energetic. I'm excited to play with Richard (Sherman), Brandon (Browner), Kam (Chancellor) and Earl (Thomas) because they're some of the most talented players on defense in the NFL right now."
Winfield, who will turn 36 this season, was released by the Minnesota Vikings last month, making him an unrestricted free agent.
He visited the Seahawks on Tuesday after meeting with the Washington Redskins last week.
Winfield could be the slot corner for the Seahawks with starters Sherman and Browner playing outside. Sherman and Browner combined for 11 interceptions last season, but no other Seattle cornerback contributed any to the team total.
Information from ESPN's Josina Anderson and Ed Werder was used in this report.