Flanker Seymour captained Sale at the start of last season.
"From watching Sale last year I could see even before getting here you could see how committed David is," said Redpath.
"We need someone in there who is going to be close to the referees and be able to give some feedback.
"He leads by example in most things he does and he's a great role model for both the other senior professionals and all of the younger guys at the club."
Sam Tuitupou has been named vice-captain.