The Minnesota Vikings will receive three draft picks, including a first-round selection this year, in return for the wide receiver.
The reigning league MVP tweeted: "The best all around player I ever seen or you'll ever see! Goes to Seattle! I feel like I just got kicked in the stomach. Several times!!!
"I wish my boy Percy nothing but success! God bless you homie."
The 24-year-old was the Vikings' leading receiver in yards in 2012, despite missing the second half of the campaign with an ankle injury.