The 38-year-old reliever comes to Florida from the Mariners for Minor League pitching prospect Gaby Hernandez.
"We added some depth to our bullpen," Marlins president of baseball operations Larry Beinfest said.
The Marlins were hopeful of making a bigger splash before Thursday's 4 p.m. ET non-waiver Trade Deadline.
The club was in the mix of a three-way trade for slugger Manny Ramirez, but that fizzled early Thursday afternoon. And Beinfest said the team explored about a "half dozen" scenarios for a veteran catcher.
Nothing was able to be worked out, other than adding Rhodes to one of the most heavily-used bullpens in the big leagues.
All season the Marlins had good scouting reports on Rhodes, who was 2-1 with a 2.86 ERA in 36 games with the Mariners.
The Marlins reinforced their opinion of Rhodes during an Interleague series at Seattle, June 16-18.
During that series, Rhodes' fastball was clocked in the 93-94 mph range.
Rhodes' success against the Marlins played a part in the evaluation process.
Rhodes broke into the league with the Orioles in 1991. He's logged 689 games in the big leagues.
The addition of Rhodes should help ease the workload of lefty Renyel Pinto, who has appeared in 58 games.
To acquire Rhodes, the Marlins parted with Hernandez, a promising 22-year-old from Miami who was in Triple-A early in the season before being sent down to Double-A Carolina.
"Great kid. We felt like we were dealing from an area of strength," Beinfest said of Florida's abundance of Minor League pitching. "We wanted to get the reliever in here. We wanted to add depth."
Catching depth remains a challenge.
The Marlins had explored a number of options, including talking with the Tigers regarding Ivan Rodriguez, who was dealt to the Yankees on Wednesday.
The Marlins also coveted San Francisco's Bengie Molina, Texas' Gerald Laird and Baltimore's Ramon Hernandez.
Right now, with Matt Treanor on the disabled list with a strained left hip, John Baker and Paul Hoover are handling catching duties.
The team is hopeful to get Treanor back, perhaps as early as next week.
Beinfest admitted the search for catching help was challenging.
"It's an area that there is not a lot of depth from within the game," Beinfest said. "Trading for even a backup this time of year is tough because so many teams are in it. They need to keep their backups as well, because they don't have extra inventory in the Minor Leagues.
"In some catching markets, people believe there is enough of a catching need, and they want to maximize it. They didn't feel July 31 was the time to maximize it. They wanted to wait until the winter."
Joe Frisaro is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.