They also traded cornerback Ronald Darby to the Eagles in exchange for Jordan Mathews and a 2018 third-round pick.
The trades were announced minutes apart on Twitter.
According to ESPN's Adam Schefter, the Bills have agreed to trade Watkins and their 2018 sixth round pick to the Los Angeles Rams in exchange for defensive back E.J. Gaines and the Rams' 2018 second round pick.
Buffalo Bills Verified account @buffalobills
The Rams drafted Gaines in the sixth round of the 2014 NFL Draft. Foot injuries have been his bane, causing him to miss 11 games the past two seasons. Earlier this offseason, the Bills declined the fifth-year option on Watkins' contract, making him a free agent after this season. He will be reunited in Los Angeles with Robert Woods and likely fill their number one receiver role. In his three years with the Bills, the former Clemson product has hauled in 153 passes for 2,459 yards and 17 touchdowns. He had a career best 1,047 yards and nine touchdowns on 60 receptions in 13 games during the 2015 season.
As a result of these transactions, the Bills now own two selections in the first, second and third round of the 2018 Draft.
Now the Bills will be heading in an entirely new direction at wide receiver.More news: Rogers Cup: Denis Shapovalov can't get Penny Oleksiak to play tennis