An 18-year-old man who was hit by a truck on Tuesday has died in hospital.

The Forrestdale man was riding an motorised bicycle about 11.20am when he was hit by a white Freightliner truck at the intersection of Thomas Road and Tonkin Highway in Oakford.

The cyclist was taken to Royal Perth Hospital but passed away late yesterday.

The 46-year-old truck driver from Piara Waters is assisting police with their inquiries.

Major crash investigators are asking for anyone who saw the incident or either the truck or bicycle prior to the crash to contact Crime Stoppers on 1800 333 000.