There have been at least three reported thefts of motorbikes in Weymouth since Thursday 6 April 2017 and a further three on Portland.
Two of the bikes stolen on Portland have been recovered but were damaged beyond repair.
Officers are now appealing for anyone with information to contact them and are also encouraging motorcyclists to make sure their vehicles are properly secured.
Police Sergeant Jane Moore, of the Weymouth Neighbourhood Policing Team, said: “I would urge anyone who sees people behaving suspiciously around motorcycles or has any information about stolen motorbikes to please contact us.
“I would also like to advise motorcycle owners to ensure their vehicles are secured at all times when unattended and, if possible, parked in a garage or secure area overnight.”
For more crime prevention advice visit www.dorset.police.uk/help-advice-crime-prevention.
Anyone with information is asked to contact Dorset Police at www.dorset.police.uk, via email firstname.lastname@example.org or by calling 101, quoting occurrence number 55170028018. Alternatively contact Crimestoppers anonymously on 0800 555111 or via www.crimestoppers-uk.org.