Cox should upgrade a run defense that allowed 142.3 yards per game last season. Cox was considered to be a top-10 pick by most draft experts, and some believed the Eagles would have to move up further to get him.
Cox, listed at 6-foot-4, 298 pounds, is a speedy, versatile lineman who can play inside or outside. He was named to the All-Southeastern Conference first team after he collected 56 tackles and five sacks last season.
The Eagles sent their first-round pick (15th), a fourth (114th) and a sixth (172) to Seattle to acquire Cox.
The Associated Press contributed to this report.