Brown, a three-time Pro Bowler who joined the Seahawks from the Pittsburgh Steelers, had been one of the Seahawks' few bright spots this season. He had a team-high seven sacks in eight games, and has 70 sacks for his career.
"It's the top of the foot, and he's going to have some hardware put in there to hold it together," said Coach Mike Holmgren.
"It's a huge loss," said safety Reggie Tongue. "He makes a lot of plays. It hurts, but what can you do? It's part of the game."
Brown played for Pittsburgh from 1993-96 before joining Seattle.
The Seahawks, who are only 2-6, have been hit hard by injuries to key players in skill positions.
No. 1 quarterback Trent Dilfer is done for the year because of a right torn Achilles tendon, wide receiver Darrell Jackson is recovering from a concussion and seizure on Oct. 27 in the Dallas game, and starting linebacker Anthony Simmons has not played since a season-opening loss to the Oakland Raiders because of a sprained knee.