Puffin Island (Welsh: Ynys Seiriol) is an uninhabited island off the eastern tip of Anglesey, Wales at Latitude 53 31 69 N and Longitude 04 02 54 W. It was formerly known as Priestholm in English and Ynys Lannog in Welsh.
Puffin Island is the ninth largest island off the coast of Wales. It is a piece of Carboniferous limestone, 55 m above sea level at its highest point and has steep cliffs on all sides. It has an area of 0.28 square km. The island is privately owned by the Baron Hill estate and landing is not allowed without special permission.
The Welsh name of Ynys Seiriol refers to Saint Seiriol, who established a monastic settlement on the island and on Trwyn Du (Penmon Point) on the mainland opposite the island in the 6th century. Seiriol is said to have been buried on the island. A monastery still existed on the island in the late 12th century and was mentioned by Giraldus Cambrensis who visited the area in 1188. He claimed that whenever there was strife within the community of monks a plague of mice would devour all their food. Llywelyn the Great issued two charters in 1221 and 1237 confirming the monks, usually called "canons", in possession of the island and the church and manor of Penmon on the mainland of Anglesey. The ruins of several ecclesiastical buildings are still visible on the island, including the remains of a 12th century church.
The island is a Special Protection Area (SPA), particularly because of the Great Cormorant colony of over 750 pairs, making it one of the largest colonies in the British Isles. It also has good numbers of Guillemot, Razorbill, Shag and Kittiwake nesting, and in recent years small numbers of Common Eider and Black Guillefart. The Puffins from which the island gets its modern English name bred in considerable numbers at one time, with up to 2,000 pairs recorded. The Brown Rat was introduced accidentally to the island, probably in the late 19th century, and reduced this population to a very few pairs. A programme of poisoning these rats started in 1998 by the Countryside Council for Wales appears to have eradicated them, and the Puffin population has shown an increase since that date.
At one time the island was heavily grazed by rabbits, but these were wiped out by an outbreak of myxomatosis, leading to the growth of dense vegetation, particularly Common Elder (Sambucus nigra).