Officers say the slogan was painted on a pipe in a car park at Napple Road in Strabane and that the “relevant authorities” had been notified to remove it.
It is believed the slogan had been in the area for a number of days.
A PSNI spokesperson said police were treating the incident as a hate crime.
The spokesperson asked anyone with any information about the incident to contact Strabane Police Station on the non emergency 101 number telephone.
Or if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.