A source said Thursday that Bell's contract with the Marlins was essentially a "done deal." The Marlins did not confirm the deal.
According to an ESPN.com report, Bell agreed to a three-year deal worth $27 million, a deal that includes a vesting option for a fourth year.
The report indicated that Bell was going to take a physical on Friday.
Bell wasn't available for comment.
The Padres offered Bell arbitration last week, presuming that he would sign elsewhere, leaving the team with two additional Draft picks because the 34-year-old was projected as a Type A free agent.
The Padres will receive a pick in the supplemental round and then an additional pick in the second round before the Marlins' second-round Draft pick.
Bell has racked up 132 saves over the past three seasons while posting an ERA of 2.36 with at least 62 innings pitched each year. He recorded 43 saves and a 2.44 ERA while striking out 51 batters and walking 21 in 62 2/3 innings in 2011.
Bell, who has more saves in the Major Leagues since the start of 2009 than any other pitcher, was hopeful that he could reach a deal to remain with the Padres. San Diego, which now is in the position of looking for a closer, didn't seem willing to go more than two years.
Bell had until 8:59 p.m. PT on Dec. 7 to accept the Padres' offer of arbitration.
Bell, who came to the Padres before the 2007 season in a trade with the Mets, had an idea as far back as Spring Training that the 2011 season could be his last in San Diego.
"Am I at peace with this? Partly yes and partly no," Bell said in June. "Guys have to make their decisions that are best for the organization. If that means trading me, I get it. I love San Diego. I love the organization. But sometimes you have to go in a different direction.
"I've had a lot of people come up and say to me that, 'Oh, maybe you'll get a chance to go play for a contending team and maybe you'll win a World Series.' Well, I wanted to win one here."
But the Padres retained Bell at the Trade Deadline, sending eighth-inning specialist Mike Adams to the Rangers for two pitching prospects, unhappy with the deals that they were presented for Bell.
San Diego general manager Josh Byrnes said earlier Thursday that finding pitchers for "the eighth and ninth innings" is at the top of the club's offseason priority list as it heads to Dallas on Sunday for the Winter Meetings, which begin Monday.