Dindaethwy was in medieval times one of two commotes of the cantref of Rhosyr, in the south-east of the Isle of Anglesey. It was between the Menai Strait and Conwy Bay (to the south), and the Irish Sea and Red Wharf Bay (to the north). It included Penmon, the easternmost point of the island, opposite which is Puffin Island (Ynys Seiriol). It bordered the commote of Menai (the other commote of Rhosyr) to the west, and the commote of Twrcelyn in the cantref of Cemais, to the north. Later, Dindaethwy was the home of Penmynydd, the family estate of the Tudors of Anglesey.