An elderly woman riding a mobility scooter has been seriously injured in a crash with a truck in Jurien Bay.

Police are investigating the incident which occurred about 9am on Friday.

Police said that a woman aged in her 90s riding a mobility scooter and a white Isuzu truck collided on Bashford Street.

The truck driver, a man aged in his 70s, gave first aid to the woman until St John WA paramedics arrived.

The woman was airlifted by a RAC Rescue helicopter to Royal Perth Hospital with life-threatening injuries.

Police urge anyone who witnessed the incident, or has CCTV, dash-cam or mobile phone vision, to contact Crime Stoppers online at www.crimestopperswa.com.au or call 1800 333 000.