St Mary's Isle is a partially submerged reef in Douglas Bay on the Isle of Man. Prior to 1832 the rock was the property of the Quane family until John Quane, Attorney General of the Isle of Man, presented the rock to Sir William Hillary, in his capacity as President of the Isle of Man District of the Royal National Lifeboat Institution to build the Tower of Refuge. Since then the custody of St Mary's Isle has been held by each president of the local lifeboat association.

Douglas is the capital city and largest settlement of the Isle of Man, with a population of 26,677 (2021) and an area of 10.5 km2. It is located at the mouth of the River Douglas, and on a sweeping bay of two miles. The River Douglas forms part of the city's harbour and main commercial port.