Police were called to Mattison Way in Greenwood, north of the CBD, about 8am today over reports a man had been injured.
Officers arrived to find a man in his 80s and a woman in her 70s both seriously injured.
The man died at the scene.
The woman was taken to Royal Perth Hospital, where she remains in stable condition.
An alert was issued for residents to remain indoors after a man was allegedly on the run armed with a knife.
“A large number of police resources were deployed to the scene including police airwing, canine unit, local police and detectives,” Western Australia Police said.
Almost an hour and a half later, a 19-year-old man was located on Forrest Hill Drive and placed into custody.
There is no longer a concern for public safety, police said, but motorists are advised to avoid the area due to ongoing traffic delays in and around the suburb.
Police inquiries remain ongoing.