The team says Tuesday it had to claim the former San Diego Chargers player last week but then waive him and allow him to clear waivers before Seattle could rework a new deal.
Ellison was an All-Pac-10 selection and team captain at USC for new Seahawks coach Pete Carroll.
He started nine games at strong safety as a rookie for San Diego last season.
The Chargers waived Ellison after police in Redondo Beach, Calif., stopped him for speeding in a school zone near his former high school in late May. Police said a search of Ellison's vehicle turned up 100 Vicodin pills.