The woman, identified as Amanda Mosley, was taken to a hospital where she later died, KPHO-TV, Phoenix, reported Tuesday.
Phoenix Sgt. Tommy Thompson said Mosley was hugging her 18-year-old boyfriend, whose name was not reported, when a handgun he was removing from his waistband went off.
The man was removing the gun because it was causing Mosley discomfort, Thompson said.
The incident appeared to be an accident, Thompson said.