In Tampa Bay, Jackson will pair with 2016 Pro Bowl wide receiver Mike Evans, who was fourth in the NFL last season with 1,321 receiving yards and tied for second with 12 touchdown catches.
"It is rare to find a player in free agency with the combination of speed and natural playmaking ability of a DeSean Jackson," said Buccaneers general manager Jason Licht in a release announcing the deal.
"DeSean is exactly the type of dynamic playmaker we have been targeting for our offense," added Buccaneers head coach Dirk Koetter. "DeSean brings a veteran presence and a big-play mentality that fits in perfectly with our offensive philosophy. He is a tough-minded competitor who has the game-breaking speed and pass-catching ability that stretches the defense and creates matchup problems."
The Redskins now have lost their top two wide receivers in free agency, with Pierre Garcon having signed a five-year deal with the San Francisco 49ers.
Jackson had spent the past three years with the Redskins, averaging an NFL-best 19.03 yards per catch during that time. He averaged 17.2 yards per catch in six seasons with the Philadelphia Eagles.
Jackson, who turned 30 in December, finished strong in 2016, with four 100-yard performances in his last six games. In that time, he averaged 24.45 yards on 22 receptions with 2 touchdowns. He finished the season with 56 receptions for 1,005 yards -- the fifth time he has surpassed 1,000 yards in a season.
He has established himself as one of the NFL's best deep-ball receivers as he uses his speed -- as well as his days playing center field in baseball -- to track the ball for big plays.
At times, Jackson appeared frustrated with a lack of downfield opportunities but would only say he doesn't call the plays. He also was a source of offseason controversy pertaining to how often he would show up for volunteer workouts.
The three-time Pro Bowl selection was released in the 2014 offseason after six seasons with the Eagles. With the Eagles, Jackson, a second-round pick in 2008, caught 356 passes with 32 touchdowns.
ESPN's John Keim contributed to this report.