Newport Beach Police Department spokeswoman Kathy Lowe said the shooting took place about 2:45 p.m. in a second-floor patient room at 520 Superior Ave., KNBC-TV, Los Angeles, reported.
Police responded to a call of as many as seven shots fired in the medical building and found both the victim and the gunman in the office, the TV station said.
Witnesses said the victim was a urologist. Police did not release the name of the victim or the gunman, who was arrested without incident, or provide details about his relationship to the doctor.
The Los Angeles Times reported several people said the victim was a doctor at Orange Coast Urology and workers in the building said the shooting occurred in the doctor's exam room.
"I sit right at the front desk; I would have caught the first bullet," Becky Calderwood, who works two doors down from the office where the shooting occurred, told the Times. "This is nuts; people are just shooting everyone all the time."
Calderwood said the gunman was a 70-year-old patient, and the Times said police led a handcuffed older man wearing a baseball cap out of the building.