The shooting took place in the parking lot of Club Blu just after 12:30 a.m. ET and police found "several victims suffering from gunshot wounds," the Fort Myers Police Department said in a statement. Injuries ranged from minor to life-threatening.
Three people had been detained for questioning, the police said, and the area was "deemed safe" although roads in the vicinity were closed as the investigation continues.
Local television reported that the club was hosting a "teen night."
The shooting comes six weeks after a massacre at a nightclub in the Florida city of Orlando, in which a lone gunman killed 49 people in the deadliest mass shooting in U.S. history.