The 26-year-old receiver had 182 receptions for 2,123 yards and 11 touchdowns in four seasons with the Arizona Cardinals, including a 759-yard season in 2012.
Roberts can play inside or outside and was productive when slotted into the starting role in Arizona.
Teams can sign free agents Tuesday at 4 p.m. ET.
UPDATE:The Redskins have signed Roberts, the team announced Wednesday.
In the latest edition of the "Around The League Podcast," the guys study the free agency picture with the help of NFL Media Insider Ian Rapoport.