The contract includes $75 million guaranteed and is worth $110 million over the five years of the deal, sources said. It's the highest amount of guaranteed money ever given to an NFL wide receiver.
In a video tweeted out by the Rams, Kupp expressed his excitement over the deal.
"Really glad to be coming back to Los Angeles and be here in the long haul," he said. "This is going to be an incredible ride. Can't wait to get back at it."
Last season, Kupp led the league with 145 catches for 1,947 yards and 16 touchdowns during the regular season. He also caught two touchdowns in Super Bowl LVI and was named MVP of the game. He became the fourth player since the 1970 merger to lead the league in receptions, receiving yards and receiving touchdowns.
After the Rams' first practice of minicamp on Tuesday, Kupp said he was not looking for a contract that would reset the wide receiver market.
"I'm not trying to compare myself," Kupp said. "I'm not trying to say, 'OK, well where was Tyreek [Hill] at? All those guys that got deals, where were all those guys at? And I need to be higher than them in certain places.'"
Kupp also said Tuesday that he wanted the contract to be "something we can work on together."
"Something that's great for the team, something that's great for me and my family as well," Kupp said. "That place exists, and it's just getting there."
On Monday, the Rams reworked defensive tackle Aaron Donald's contract to make him the highest-paid non-quarterback in the NFL. Los Angeles also extended quarterback Matthew Stafford and signed linebacker Bobby Wagner and wide receiver Allen Robinson II this offseason.
Since the 1970 merger, Kupp is one of eight players with 5,500 receiving yards and 40 receiving touchdowns within his first five career seasons. The others are Mike Evans, A.J. Green, Calvin Johnson, Larry Fitzgerald, Randy Moss, Marvin Harrison and Jerry Rice.
ESPN Stats & Information research contributed to this story.