Saint Ia of Cornwall (also known as Eia, Hia or Hya) was a Cornish evangelist and martyr of the 5th or 6th centuries.
She is said to have been an Irish princess, the sister of Saint Erc. A spiritual student of Saint Baricus, Ia travelled as a missionary to Cornwall where she joined Saints Fingar and Piala. Legend holds that they had up to 777 companions and that she sailed across the Irish Sea on a leaf which miraculously grew into a boat. Ia was martyred on the River Hayle and buried at what is now St Ives, where St Ia's Church—of which she is now the patron—was erected over her grave. The town built up around it. Her feast day is 3 February.
She is also mentioned in the Life of Saint Gwinear as a companion who followed Gwinear from Brittany to Cornwall, where she was martyred under King Teudar (i.e. Tudur Mawr, ruler of Penwith). The church of Plouyé in Brittany was probably dedicated originally to this saint. John Leland gives details from a Latin life of Ia which is no longer extant.