Dunn's sports agency tweeted an announcement of the deal Sunday, and the Marlins confirmed an agreement.
Dunn went 10-6 with a 3.16 ERA in 57 innings for Miami last year, all in relief. This season will be his fifth with the Marlins, and he has a career ERA of 3.36.
In other Marlins news, general manager Dan Jennings told SiriusXM MLB Radio that offseason trade acquisition Dan Haren is ready to be part of the Marlins' rotation.
"We've talked about his desire to pitch out on the West Coast," Jennings said. "We've had conversations with him. He is preparing for spring training like he's done any other year. I think he will be primed and ready to go. To use his own words, he's looking very forward to being a Miami Marlin."
The 34-year-old Haren, who was acquired from the Los Angeles Dodgers as part of the Dee Gordon trade in December, stated his desire after the trade to pitch for a team near his West Coast home. Most of Haren's 12-year career Haren has occurred near the West Coast. In addition to the Dodgers, he has pitched for the Los Angeles Angels, Oakland A's and Arizona Diamondbacks.
Haren registered a 13-11 record and 4.02 ERA in 186 innings across 32 starts for the Dodgers in 2014.
Information from The Associated Press was used in this report.