Police say an 18-year-old Perth man has died after jumping into the Swan River from cliffs at Blackwall Reach in Bicton.

Emergency services were called to the scene on Friday afternoon by the man's friends, who said they saw him jump into the water just after 5:00pm but failed to surface.

"Once in the water the youth got into difficulties and his friends attempted to assist him," a police spokesman said.

Water Police and St John Ambulance attended and a police diver was deployed.

The teenager, from the suburb of Dianella, was pulled unconscious from the water onto a police boat and rushed to Fiona Stanley Hospital shortly after 6:00pm.

Attempts to resuscitate him were unsuccessful and he was declared dead a short time later.

Police said the death was a tragic accident and was not being treated as suspicious.

It is expected there will be a coronial inquiry.

The limestone cliff face has long been a popular spot for diving into the deeper water of Swan River, although signs posted in the nearby reserve warn against jumping into the water.

Earlier this year a Taiwanese tourist died after making the 10-metre plunge.