An unidentified source at Miami's Florida Room nightclub said the 30-year-old Hollywood star showed no signs of his recent hospitalization while out enjoying himself Thursday night, People.com reported Friday.
"Josh jumped on stage with a local Miami band," the source said. "He looked happy and was having fun."
The Miami nightclub appearance by the "Hollywood Homicide" star came a day after Hartnett was seen with friends at the Chateau Marmont hotel in Los Angeles.
The famed hotel where Hartnett was spotted Wednesday night was the same locale Hartnett was rushed from March 30 after he complained of serious stomach pains, People.com said.
Hartnett, whose other movies include "Black Hawk Down" and "The Black Dahlia," was released from the hospital April 3.