Jackson's publicist tweeted that Jackson was signing with the Redskins following two days of meeting with coaches and officials.
Jackson also broadcast the news, tweeting: "ITS GOIN DOWN !! BURGUNDY & GOLD".
The deal is worth $24 million, including $16 million guaranteed, sources told ESPN.
Jackson arrived in the D.C. area Monday night and spent much of Tuesday at Redskins Park while his agent negotiated with the front office.
Jackson set career-highs with 82 catches for 1,332 yards last season for the NFC East champion Eagles, but Philadelphia tried to trade him and then cut him last week amid reports of off-the-field concerns. Jackson issued a statement denying he was associated with street gangs.
The Redskins, coming off a 3-13 season, had been looking for a versatile threat to add to an offense that includes Robert Griffin III, Pierre Garcon, Andre Roberts, Jordan Reed and Alfred Morris.
Jackson becomes the second player the Redskins have lured from an NFC East rival this offseason. Defensive lineman Jason Hatcher signed with Washington after eight seasons with the Dallas Cowboys.
Jackson's signing is the first headliner move since the hiring of coach Jay Gruden and since general manager Bruce Allen was put in charge of the roster, but it echoes a familiar pattern from past years under Redskins owner Dan Snyder: Go after the hot name and live with whatever baggage there might be. The strategy has often failed - Albert Haynesworth and Donovan McNabb are recent examples - but the 27-year-old Jackson is still in his prime.