Jackson was cut by the Eagles last week after reports linked him to a Los Angeles-based gang.
The 27-year-old was a key part of the Eagles' offense last season, chipping in with career-high figures of 1,332 receiving yards and nine touchdowns.
Despite both his excellent form and his denial to the gang link, the Eagles cut all ties with Jackson, allowing him to seek pastures new.
With nine teams in the hunt for Jackson's signature, it is the Redskins who have won the race, striking a four-year $32m (£19.2m) deal on Wednesday.
Jackson had played his entire NFL career up to this point with the Eagles, since being drafted by the franchise back in 2008.