Safety Donte Whitner, an Ohio native, signed a four-year, $28 million offer from the Cleveland Browns, according to the Contra Costa Times.
Whitner, 28, was an unrestricted free agent and became the first player from the 49ers' 2013 roster to sign elsewhere.
Following the 2011 labor lockout, Whitner nearly accepted a two-year offer from the Cincinnati Bengals before Jim Harbaugh phoned him with a winning sales pitch that led to Whitner accepting a three-year, $11.7 million deal with the 49ers.
C.J. Spillman and Craig Dahl are the top, in-house options to replace Whitner and pair with second-year safety Eric Reid.
"The business side to the league is never fun," Reid posted on Twitter. "Hate to see him go but happy for my man @DonteWhitner. Appreciate everything you did for me."
Three of the 49ers' other defensive backs also hit the free-agent market: cornerbacks Tarell Brown, Eric Wright and Perrish Cox. Meanwhile, kicker Phil Dawson announced on Twitter that he's re-signing with the 49ers.