The shooting happened around 11:55 p.m. Tuesday at Methodist Northlake Hospital, 600 Gordon Hatcher Blvd, in Gary, Indiana, police said.
Methodist Northlake Hospital said a patient in the waiting room had displayed a firearm. The security staff responded to the report of an active shooter, "neutralizing the threat."
The suspect, an adult male whose exact age was not known, was shot by an off-duty officer, Gary police said.
They were taken to a different hospital to receive medical treatment, Methodist Northlake said. The man's condition was not released.
"Methodist Hospitals is cooperating with local law enforcement as the investigation continues," a statement from the hospital read in part. "Weapons of any kind are prohibited at all Methodist Hospitals facilities. The safety of our patients and employees remains our top priority."
The Lake County Sheriff's Department is investigating the police-involved shooting.
"In accordance with department policy and procedure, the involved personnel has been placed on leave pending the outcome of the investigation," Gary police said in a statement.
No further information was immediately available.