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Island in Pembrokeshire, Marloes and St Brides, Wales

Skomer (Ynys Sgomer) or Skomer Island is an island off the coast of Pembrokeshire, in the community of Marloes and St Brides in west Wales. It is well known for its wildlife: around half the world's population of Manx shearwaters nest on the island, the Atlantic puffin colony is the largest in southern Britain, and the Skomer vole (a subspecies of the bank vole) is unique to the island. Skomer is a national nature reserve, a Site of Special Scientific Interest and a Special Protection Area. It is surrounded by a marine nature reserve and is managed by the Wildlife Trust of South and West Wales. Skomer is known for its archaeological interest: stone circles, standing stone and remains of prehistoric houses. Much of the island has been designated an ancient monument.

The island has an area of Its highest point is above sea level at Gorse Hill, while the majority of the island sits at around above sea level. Skomer is intersected by a series of slopes and ridges giving it a rich and varied topography. It is approximately from north–south and east–west. The island is almost cut in two near its eastern side by two bays. It is one of several islands lying within 1000 yards (a kilometre) of the Pembrokeshire coast. A number of islets surround Skomer, the largest of which are: Midland Isle (height ) separated from Skomer by Little Sound, Mew Stone (height ) and Garland Stone (height ).

The name Skomer derives from Skalmey, a name of Viking origin meaning "Cleft island", possibly from the fact that the eastern end of the island is nearly cut off from the main part. It is marked on a 1578 map in Latin as Scaline Insul, with the first word probably meaning scalene or unequal.

The volcanic rocks of which Skomer is comprised date from the Silurian period around 440 million years ago. A series of basalts, rhyolites, felsites, keratophyres, mugearite and associated sedimentary rocks (quartzites, etc.) are grouped together as the 'Skomer Volcanic Series'. The series which is up to 1000 metres (3000') thick also includes trachyte, dolerite and skomerite which is an altered andesite. Basalt is the most common component of this sequence; some of it appears as pillow lava indicating that it was erupted under water. Other basalt flows show signs of contemporary subaerial weathering. This same suite of rocks can also be traced eastwards on the mainland along the northern side of the Marloes peninsula and extends almost as far east as St Ishmael's. The entire sequence on Skomer dips between 15° and 25° to the south southeast. It is cut by several faults, notably those responsible for the erosion of the inlets of North Haven and South Haven. A NW SE aligned fault stretches between Bull Hole and South Haven, offsetting the strata on either side. Skomer was cut off from the mainland by rising sea levels after the last Ice Age.

There is evidence of human occupation—field boundaries and settlement remains—dating back to the Iron Age. The Skomer Island Project, run jointly by the Royal Commission for Ancient and Historical Monuments in Wales (RCAHMW) with archaeologists from the University of Sheffield and Cardiff University, started in 2011, investigates the island's prehistoric communities. Airborne laser scanning together with ground excavations continued in 2016 and established that human settlement dates back 5,000 years. Rabbits were introduced in the 14th century and their burrows and grazing have had a profound effect on the island landscape. The farm buildings in the centre of the island, now housing visitor accommodation, were refurbished in 2005. In 1963, the Look at Life documentary series featured Skomer and its nature, and Saunders' role as warden and his home life with his wife and family.

Skomer is best known for its large breeding seabird population, including Manx shearwaters, guillemots, razorbills, great cormorants, black legged kittiwakes, Atlantic puffins, European storm petrels, common shags, Eurasian oystercatchers and gulls, as well as birds of prey including short eared owls, common kestrels and peregrine falcons. The island is also home to grey seals, common toads, slow worms, a breeding population of glow worms and a variety of wildflowers. Harbour porpoises occur in the surrounding waters. The Skomer vole, a subspecies of bank vole, is endemic to the island.

There are around 10,000 breeding pairs of puffins on Skomer and Skokholm Islands, making them one of the most important puffin colonies in Britain. They arrive in mid April to nest in burrows, many of which have been dug by the island's large rabbit population. The last puffins leave the island by the second or third week in July. They feed mainly on small fish and sand eels; often puffins can be seen with up to a dozen small eels in their beaks. After a period of declining numbers between the 1950s and 1970s, the size of the colony is growing again at 1–2% a year (as of 2006). By 2004, there were numerous puffin burrows on the island and adults flying back with food run across the walkways oblivious to the tourists. A 2019 survey estimated a population of 24,108 birds, and in 2020 this had increased to 34,796 birds.

Skomer has one unique mammal: the Skomer vole (Myodes glareolus skomerensis), a distinct subspecies of the bank vole. The lack of land based predators on the island means that the bracken habitat is an ideal place for the vole, with the population reaching around 20,000 during the summer months. Then the resident short eared owls may be seen patrolling the areas close to the farmhouse in the centre of the island for voles to feed their young.

The Dale Princess sails to Skomer from Martin's Haven on the mainland, a sheltered 15 minute trip every day (weather dependent) except Monday (Whitsun Bank Holiday Monday excepted) from April to September between 10am and noon (actual times may vary). There are limits on the number of people allowed to visit the island (250 per day). Advance booking is recommended after the introduction of a new visitor management system in 2020. A self catering hostel with 16 beds is situated at the farm in the centre of the island. Booking opens in October and can only be done by phoning the Tondu office. Overnight guests are brought over on a separate boat trip on the morning of their stay and the hostel is open April to September. Areas open for visitor access are restricted to pathways. The Neck, an eastern area connected only by a narrow isthmus, is entirely out of bounds to visitors. In 2005–06, there was a renovation project of the farm buildings which included the old barn for improved overnight visitor and research accommodation, the volunteers' quarters were rebuilt and the warden's house at North Haven was also rebuilt. Solar power provides hot water and electricity for lighting.