Rivers was one of only two first-round draft picks still unsigned. The linebacker from Southern California showed up at the team's training facility on Tuesday night and signed the deal. Terms of the deal weren't disclosed.
The signing leaves Jacksonville defensive end Derrick Harvey, taken eighth overall, as the final first-round holdout.
"He's behind, but we'll work to get him up to speed, and we know he's the kind of player who will get himself caught up," coach Marvin Lewis said. "Right from the start, through the draft and minicamps, he has shown us things that are innate to successful NFL linebackers."
Rivers was the ninth overall pick in the draft. The Bengals are hoping that he helps fix their problems on defense, which have dragged them down for years.
"I'm ready to pick up where I left off," Rivers said. "I want to catch up quick on what I missed and move forward as fast as I can."
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