Officers said the posters have been found throughout Durham between October 2019 and February 2020.
According to Durham police, all of the posters have white backgrounds with a black text and all have similar anti-immigration messages.
Police said the most recent incidents occurred in early February in Whitby. They said posters were found on the Whitby Public Library, a school in Brooklin and on a mailbox.
Investigators said similar posters were found in October in Port Perry and in Newcastle back in November.
Police are asking anyone with information about these incidents to call them at 1-888-579-1520 ext 1871 or anonymously at Durham Regional Crime Stoppers at 1-800-222-8477.