Officers with the North Wales Police Rural Crime Team say the spraying at Pontcysyllte Aqueduct, near Chirk, happened some time between September and November.
A spokesperson for the rural crime team said: “Sometime between September and November 2023 unknown person(s) have sprayed graffiti on the Pontcysyllte Aqueduct which is not only a schedule one monument but also a UNESCO World Heritage site.
“Anyone with information as to who the tag may belong to or has any information that would assist with enquiries is asked to contact North Wales Police and quote reference 23001117304.”