Surtsey ("Surtr's island" in Icelandic, Icelandic pronunciation: ​[ˈsʏr̥(t)sˌeiː]) is a volcanic island located in the Vestmannaeyjar archipelago off the southern coast of Iceland. At 63.303°N 20.605°W / 63.303; -20.605 Surtsey is the southernmost point of Iceland. It was formed in a volcanic eruption which began 130 metres (430 feet) below sea level, and reached the surface on 14 November 1963. The eruption lasted until 5 June 1967, when the island reached its maximum size of 2.7 km² (1.0 sq mi). Since then, wave erosion has caused the island to steadily diminish in size: as of 2012, its surface area was 1.3 km² (0.50 sq mi). The most recent survey (2007) shows the island's maximum elevation at 155 m (509 ft) above sea level.

The new island was named after Surtr, a fire jötunn or giant from Norse mythology. It was intensively studied by volcanologists during its eruption, and afterwards by botanists and other biologists as life forms gradually colonised the originally barren island. The undersea vents that produced Surtsey are part of the Vestmannaeyjar submarine volcanic system, part of the fissure of the sea floor called the Mid-Atlantic Ridge. Vestmannaeyjar also produced the famous eruption of Eldfell on the island of Heimaey in 1973. The eruption that created Surtsey also created a few other small islands along this volcanic chain, such as Jólnir and other, unnamed peaks. Most of these eroded away fairly quickly. It is estimated that Surtsey will remain above sea level until at least the year 2100.