An event or combination of events caused by agricultural pollution have impacted the water quality at the beach.
The council is advising members of the public against bathing, with the situation set to last up to 72 hours.
An Isle of Anglesey County Council spokesperson said: “An event or combination of events impacting on bathing water quality at Cemaes.
“We are currently advising against bathing.
“This has been caused by agricultural pollution and is not expected to last longer than 72 hours.”