Winfield, 36, was signed as a free agent in the offseason. He's starting his 15th NFL season after spending the past nine years at Minnesota, being voted into the Pro Bowl three times.
The Seahawks planned to make Winfield their nickelback, but he was a victim of numbers in what may be the deepest secondary in the NFL.
Cornerback Walter Thurmond, who was believed to be on the bubble for making the roster, had an interception in the final preseason game Thursday night against Oakland.
Winfield had signed a one-year deal with Seattle after Minnesota released him from a contract that would have paid him $7.25 million in 2013.