Peterson, who will turn 35 in March, was Washington's leading rusher in each of the last two seasons.
"Adrian Peterson is the epitome of what it means to be a pro in this league," head coach Ron Rivera said in a statement. "Adrian's leadership and passion toward the game of football will set an example of what is expected of the players in this program moving forward."
The future Hall of Fame runner signed with the Redskins prior to the 2018 campaign after being released by the Arizona Cardinals. He quickly proved to be a useful commodity in the wake of Derrius Guice's knee injury.
Peterson is a seven-time Pro Bowler and a four-time first-team All-Pro selection who ranks fifth in league history with 14,216 career rushing yards.
He'll likely split carries with Guice in 2020, his 14th NFL season.