The precautionary notice was issued by Irish Water to a large number of customers in the town and to those on nearby group water schemes this evening.
It will affect thousands of homes and businesses in the busy tourist town.
Irish Water says the HSE has issued the precautionary boil notice, after eight people in the Westport area reported symptoms of crytosporidium.
They say no crytosporidium had been detected in ongoing water samples, however, as a precaution they are urging customers to boil water before using it for drinking, preparing food and baby food or brushing teeth.