Peterson, who was playing in just his second game after missing the previous four with a high ankle sprain, was helped off the field after taking a helmet to the knee from Redskins' safety DeJon Gomes on a three-yard run up the middle.
"He has a sprained knee and were going to evaluate his condition once we get home," Vikings head coach Leslie Frazier said of Peterson's injury.
Peterson had rushed for 38 yards and a touchdown before leaving the game.
Vikings quaterback Christian Ponder also suffered a concussion on the next play after taking a hard hit in the third quarter and will not return to the game. Ponder was 8-of-13 for 68 yards before exiting the game.
"Suffered concussion-like symptoms, so by league rule we had to do some things protocol wise," Frazier said. "He'll be evaluated further once we get home as well."