The Rams traded their first- and sixth-round picks in the 2018 draft to the New England Patriots for Cooks and a fourth-round pick earlier this year.
Cooks was entering the final season of his contract and is happy to be staying in Los Angeles for the next several years.
''I just knew this was a place that I want to be for a really long time,'' he said. ''As far as the years and all of that, I just let them handle that, my agent handle that, and just knowing I wanted to be here for a really long time.
''They took care of that in the way they thought was best and we agreed with it. Other than that I'm just excited to get Game One in, let alone years, so that's what I'm focused on now.''
Before joining the Rams, Cooks played in 58 regular-season games with 46 starts, and had at least 1,000 yards receiving in each of the past three seasons.
The fifth-year pro has 280 receptions for 3,943 yards and 27 touchdowns, and averages 14.1 yards per reception. He had nine 100-plus yard receiving games in 2016 with the New Orleans Saints.
''Brandin Cooks has shown himself to be a class act on and off the field since the first day he joined our team,'' Rams coach Sean McVay said in a statement. ''He's a proven professional in this league, and signing him to a long-term contract was always our goal. We're excited to keep Brandin in a Rams uniform through 2023.''
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