"For the next three or four years, I will be playing football and basketball at the University of Oregon," said Armstead, to a throng of supporters in Sacramento.
The five-star prospect, ranked #2 overall nationally, #1 in the West and #1 at offensive tackle, chose the school after taking all five of his official visits.
Armstead was at one point committed to USC for more than a year before opening things up in the fall.
It's the second year in a row the Ducks have landed the top player in the West and a top three player nationally, a year after swiping prospect DeAnthony Thomas in the 11th hour.
And it is also the second year in a row the Ducks picked up a longtime USC commit, with Thomas being a Trojan verbal for some time.
While Scout has him projected as an offensive lineman, Armstead's plan is to play on the defensive line, which he'll do next fall for the Ducks.
And that is no problem for Chip Kelly, who turned Thomas, ranked as the #1 cornerback in the country a year ago, into the Pac-12 Freshman of the Year as a running back and return specialist.
"I'm going to play defensive line," said Armstead. "They are going to run a 3-4, which was also big for my decision. I see myself fitting in with the team and working with Coach Azzinaro. Hopefully he can turn me into an NFL draft pick."
The near-departure of Kelly to Tampa Bay last week didn't faze Armstead either.
"He was honest with me," said Armstead. "I feel like the way Oregon approached this process, they were the most straight forward and honest with me. They never bad talked any other program. They simply explained and expressed a need for me in their program and how I would fit in. They pretty much recruited me without recruiting me. They just laid everything on the table and spoke the good about their program. I was happy with how I see myself fitting in there."
CalSportsDigest.com's Dan Greenspan contributed to this report