Ynys Dulas (Dulas Island) is a small island, located off the north-east coast of Anglesey (Ynys Mon), Wales. It is the most eastern part of the parish of Llaneilian.
The island is situated about a mile and a half offshore, within Dulas Bay. The size of the island depends on the tide though it has a maximum length of roughly 600m (660 yds) and has a maximum width of 180m (200 yds). Seals are often spotted living on and around the island (hence it is also known locally as Seal Island) but it is too small for human inhabitation. There is also very little flora on the island as it is made up of rock. However on lower lying parts of the island, exposed at low tide seaweeds and other sea plants live. Two small rocks called Garreg Allan (The Outer, Expelled or Furthest Stone) are found about 100m behind the island but are not visible with the naked eye from the shore.
A raised shelf of seabed about 1.5km long reaches out a little beyond Garreg Allan, meaning that the sea around it is no more than 5 metres deep. This is followed by a drop to water much deeper (over 20 metres deep) which indicates that Ynys Dulas may have been part of a recently (geologically speaking) submerged headland.
Upon the island is cylindrical structure with a cone shaped top, built in 1824 by the Lady of Llysdulas Manor to store food and provide shelter for shipwrecked seamen. There is evidence from a map drawn up in September 1748 by Lewis Morris that the island was once known not as Ynys Dulas but Ynys Gadarn (Strong or Mighty Island).