Mannion signed with Seattle's practice squad Friday, Brady Henderson of ESPN.com reports.
Mannion is the No. 3 quarterback in the organization behind Geno Smith (groin) and Drew Lock, who are both on the 53-man roster. After spending most of the season on Minnesota's practice squad, Mannion was cut late in November after falling to fourth on the Vikings' organizational quarterback depth chart behind trade acquisition Joshua Dobbs, Nick Mullens and Jaren Hall.