Police say a cyclist wasn't wearing a helmet when he had a serious crash in Southern Tasmania early this morning.

The 24-year-old male suffered severe head injuries in the incident on Blessington Street at South Arm around 3 o'clock.

He's currently undergoing medical treatment at the Royal Hobart Hospital

Investigators are looking into what caused the crash, but say it's a timely reminder about the need for proper safety equipment.

"Police want to remind all cyclists of the importance of wearing a correctly fitted helmet," a statement said.

Anyone with information or dash cam footage is urged to contact authorities.