The team announced the move Monday to pick up the option worth about $9 million. The deal is guaranteed for injury and will become fully guaranteed next March.
Armstead was taken 17th overall in 2015. He has six sacks and 56 tackles in 30 career games. Armstead played just six games last year before a season-ending broken hand.
The Niners signed seven undrafted free agents: WR Steven Dunbar, TE Ross Dwelley, QB Jack Heneghan, OL Alan Knott, OL Jamar McGloster, DL Niles Scott, and S Terrell Williams.
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