Unst (/ˈʌnst/; Norn: Ønst) is one of the North Isles of the Shetland Islands, Scotland. It is the northernmost of the inhabited British Isles and is the third-largest island in Shetland after Mainland and Yell. It has an area of 46 sq mi (120 km²).

Unst is largely grassland, with coastal cliffs. Its main village is Baltasound, formerly the second-largest herring fishing port after Lerwick and now the location of a leisure centre and the island's airport. Other settlements include Uyeasound, home to Greenwell's Booth (a Hanseatic warehouse) and Muness Castle (built in 1598 and sacked by pirates in 1627); and Haroldswick, location of a boat museum and a heritage centre.

There are three island names in Shetland of unknown and possibly pre-Celtic origin: Unst, Fetlar and Yell. The earliest recorded forms of these three names do carry Norse meanings: Fetlar is the plural of fetill and means 'shoulder-straps', Ǫmstr is 'corn-stack' and í Ála is from ál meaning 'deep furrow'.

However, these descriptions are hardly obvious ones as island names and are probably adaptations of a pre-Norse language. This may have been Pictish but there is no clear evidence for this. Taylor (1898) has suggested a derivation from the Old Norse Ornyst meaning 'eagle's nest'.

The Shetland Amenity Trust's "Viking Unst" project excavated and displayed part of the island's Norse heritage. Work was undertaken on three longhouses – of which 60 are known to be on the island – at Hamar, Underhoull and Belmont. The replica Viking ship Skibladner can currently be seen ashore at Haroldswick.

The remains of pre-12th-century Christian chapels survive on Unst: St Olaf's Chapel, Lund, and Our Lady's Kirk at Framgord, Sandwick on the south east coast. Norse-style cross-shaped gravestones stand in the surrounding burial grounds at both Lund and Framgord, and rare "keelstone" burial markers survive at Framgord. Late Norse longhouses have been identified around both bays; the house at Sandwick still retains its cow-shaped byre door.

James Hepburn, 4th Earl of Bothwell sailed to Shetland after the Battle of Carberry Hill. He was at the house of Olave Sinclair, the receiver or sheriff of Shetland on Unst, in July 1567 when his enemies arrived in three ships, and he fought a sea battle for three hours before sailing to Norway. A later sheriff, Laurence Bruce, built Muness Castle in 1598.

The Rev Dr James Ingram (1776–1879) was minister of Unst from 1821. In the Disruption of 1843, he and most of the Unst population, left the established church and joined the Free Church of Scotland (a very typical pattern in the Highlands and Islands). He erected a new church at Uyeasound, funded by the Countess of Effingham. Ingram retired in 1875 aged 99 and died aged a remarkable 103. His father and grandfather also lived to over 100.