Both teams announced the transaction on Friday afternoon.
Watkins, 24, is a great offensive weapon when healthy. He has 17 career touchdowns in 37 games. Last season he had 28 receptions for 430 yards and two scores in eight games for the Bills. He had a career best 1,047 yards and nine touchdowns on 60 receptions in 13 games during the 2015 season. His rookie campaign included a 65 catch effort with 982 receiving yards and six scores.
The Rams sent cornerback E.J. Gaines and a second-round selection from the 2018 NFL Draft to the Bills in the exchange. The Rams also received a sixth round pick in the 2018 NFL Draft in the trade.
"Sammy has proven he can be a productive player and an explosive receiver in this league, we look forward to having him in a Rams uniform," Rams general manager Les Snead said in a team statement. "We want to thank E.J. for all he has contributed to our organization since we drafted him in 2014," added Snead. "E.J. has been a dedicated professional during his time with us and we wish him and his family the best in their future endeavors."
The Rams added Bills wide receiver Robert Woods in free agency this offseason and drafted Cooper Kupp as a possible third option for quarterback Jared Goff. Tavon Austin is currently dealing with a hamstring injury.