Hermetray (Scottish Gaelic: Theàrnatraigh) is an uninhabited island off North Uist, in the Outer Hebrides of Scotland.
Hermetray lies in the Sound of Harris on the edge of the Minch. It is on the south side of the sound although it is in the parish of Harris. The island is 72 hectares (0.28 sq mi) in area, and 35 metres (115 ft) at its highest point, Cnoc a' Chombaiste (compass hill). The bedrock of the island is Lewisian gneiss.
The hillock of Cnoc a' Chombaiste is found to the northeast. The other main geomorphological features are Acarseid Mór (big harbour) to the west a "shallow and weedy inlet with a rocky shore" and Loch Hermetray at the island's centre. There are numerous surrounding islets. The largest is Vaccasay or Bhacasaigh (island with a bay) which extends to 29 hectares (72 acres) and lies to the west. Along with Fuam to the south and Hulmatraigh to the north this island encloses the sheltered Basin of Vaccasay. Fuam's full name in Gaelic is Fuam an Aon Fhoid, meaning "far out isle of the one peat" a reference to its shallow soils. Greineam and the skerries of Staffin and Creag na Stead lie to the north in the Grey Horse Channel and Grodaigh and the once fortified Dun Mhic Leathann to the south, beyond the Seòlaid na h-Eala and close to North Uist. This strait is named after the Eala Bhàn (white swan), a famous 17th century birlinn.Seòlaid means a sailing channel, "fairway in the sea" or anchorage.