The school's website announced that Armstead filed paperwork to make in official Tuesday. The 6-foot-8, 290-pound junior said in a statement that he's ready to test himself at the next level.
Armstead started all 13 games he appeared in this season for the Ducks, and was second on the team with 5 1/2 tackles for loss. In Monday's national championship loss to Ohio State, he had a career-high nine tackles and recovered a fumble by Buckeyes quarterback Cardale Jones that led to a third-quarter field goal.
Armstead is the first Oregon player with eligibility left to declare for the draft, scheduled to start April 30. Other Oregon players who were considering leaving early for the pros include quarterback Marcus Mariota and running back Byron Marshall.