He jumped off the stage during a short performance at the LIV nightclub early Thursday morning and confronted an audience member who was heckling him. Security guards dragged him back on stage and escorted him away.
It's unknown whether the confrontation - and his pledge to quit acting and start rapping - was real or a put-on, but actor Casey Affleck recorded the performance on camera.
In October, Phoenix's publicist confirmed he was making the career switch. In January he performed at a Las Vegas nightclub and the next month he made a strange appearance on David Letterman's "Late Show." Actor Ben Stiller lampooned that appearance at the Oscars.
Hundreds of people packed the nightclub at the Fontainebleau Miami Beach hotel after the doors opened at 10:30 p.m. Wednesday night.
Several of those in the audience said they came just to see Phoenix, who left them cooling their heels for almost four hours while disc jockeys played house and hip-hop music.
Phoenix came out on stage before 2 a.m., smoking a cigarette and wearing a disheveled dark suit, sneakers and his scraggly long hair and beard.
He began rapping to a beat played by the DJ and nodding to the music, although most of the lyrics were unintelligible. Then he responded to someone who appeared to be heckling him in the audience near the stage.
"We have a (double-expletive) in the audience," he said before jumping into the crowd. It was not immediately clear whether the two men exchanged any blows.
"I saw the guy screaming at Joaquin, and Joaquin just came down," said Jorge Lledo, 30, of Miami Beach.
Security guards swarmed the scene and dragged Phoenix away.
The bizarre spectacle struck many in the audience as the latest prank in an extended farce staged by Phoenix. Affleck, an Oscar-nominated actor who is also Phoenix's brother-in-law, is making a documentary about Phoenix's career shift.
"He knows the game he's playing, and he's good at it," Gendron said of Phoenix.