In early April, the Rams acquired Brandin Cooks in a trade with the Patriots.
On Tuesday, the team and wide receiver agreed to a five-year extension, according to ESPN's Adam Schefter.
The deal is reportedly worth $80 million, but the guaranteed money is unknown at this point.
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Last year with New England, Cooks registered 65 catches for 1,082 yards and seven touchdowns.
The Oregon State product entered the NFL in 2014 as the 20th overall pick.
The 49ers host Cooks and the Rams on Oct. 21 and play their regular season finale in Los Angeles on Dec. 30.