Skomer (Welsh: 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 2.92 km² (720 acres). Its highest point is 79 m (259 ft) above sea level at Gorse Hill, while the majority of the island sits at around 60 m (200 ft) above sea level. Skomer is intersected by a series of slopes and ridges giving it a rich and varied topography. It is approximately 2.4 km (1.5 mi) from north–south and 3.2 km (2.0 mi) 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 50 metres (160 ft)) separated from Skomer by Little Sound, Mew Stone (height 60 metres (200 ft)) and Garland Stone (height 32 metres (105 ft)).

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 1,000 metres (3,300 ft) 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.