'The Mainland') is the largest and most populous island of the Isles of Scilly, an archipelago off the southwest coast of Cornwall in England, United Kingdom.

The majority of St Mary's residents live in the western half of the island, with Hugh Town alone having a population of 1,097.

The main settlement, Hugh Town (Cornish: Tre Huw), was sold to the inhabitants by the Crown in 1949 (it had belonged to the Duchy of Cornwall — which still owns much of the rest of the island).

Other settlements on the island are Old Town, Porthloo, Pelistry, Trenoweth, Holy Vale, Maypole, Normandy, Longstone, Rocky Hill and Telegraph.

Fixed-wing aircraft services are scheduled, operated by Isles of Scilly Skybus, to and from Land's End, Newquay, and (except in winter) Exeter.

Further slipways and small quays exist around the island, at natural harbour sites, such as at Old Town, Pendrathen, Watermill Cove, and Porthloo.

The lifeboatmen of St Mary's have earned 26 RNLI medals for bravery, comprising one gold, nine silver and 16 bronze, the last awarded in 2004.

Captain James Williams White (1826-1903), the champion Cornish wrestler,[11] was born on St Mary's and after emigrating lived in Burra, South Australia, from 1856.

Note: To strengthen Scilly's defences Major Daniel Lyman proposed the construction of three gun towers in 1803, each armed with a 32-pounder carronade on top.

The presence of three towers on St Mary's, in the heart of the Garrison, on Buzza Hill and at Newford Down, has led some writers to link them to Lyman's proposal.

The Star Castle was built in 1593 by the Surveyor of the Royal Works and mapmaker Robert Adams (d.1595), under the direction of Francis Godolphin, following the Spanish Armada of 1588.

[17] Fearing another Spanish invasion, in May 1593 Queen Elizabeth I ordered the construction of a fort and two sconces as a lookout for any intruder ships.

[23] Porthellick Cove contains a memorial to mark the spot where the body of Admiral Sir Cloudesley Shovell was washed ashore after the 1707 Scilly naval disaster.

This was one of the greatest maritime disasters in British history, when four ships (Association, Firebrand, Romney and Eagle) with nearly 2,000 sailors were lost.