Grimsay (Scottish Gaelic: Griomasaigh) is a tidal island in the Outer Hebrides of Scotland.
Grimsay is the largest of the low-lying stepping-stones which convey the Oitir Mhòr (North Ford) causeway, a five mile arc of single track road linking North Uist and Benbecula via the western tip of Grimsay. Until it opened in 1960, a ferry linked Carinish (on North Uist) with Gramsdale (on Benbecula), but could only operate at high tide. There was also a ford which could only be crossed close to low water, usually only with a guide. For significant parts of each day the North Ford was too wet to ford and not wet enough to cross by ferry. East of Grimsay lie several smaller islands including Ronay which was inhabited until the 1920s.
The main settlements are Baymore (Bàgh Mòr) and Kallin (Ceallan) at the eastern end of the island. Grimsay has a harbour at Kallin, which is the base to a sizeable shellfish industry, the island's main industry, mostly for lobster, prawns and scallops. Also in Kallin is The Boatshed, a marine repair facility which promotes traditional skills and includes a museum. Boat building was previously important. Grimsay is encircled by a single-track road that links most of the island's small croft and fishing settlements together.
There is a fine example of an Iron-Age wheelhouse on the northeast coast of the island at Bagh nam Feadag (The Grimsay Wheelhouse).