A small rocky island in the Firth of Forth, Fidra lies 300m off Yellowcraig on the East Lothian coast, 2 miles (3 km) north of Dirleton and 2½ miles (4 km) WNW of North Berwick. There is a lighthouse and the remains of a chapel (St. Nicholas). The lighthouse was built in 1885 by David and Thomas Stevenson and automated in 1970. Described in Robert Louis Stevenson's novel Catriona, Fidra is said to have been his inspiration for Treasure Island, although this claim is shared by Unst in Shetland.
Fidra is geologically part of a basalt sill injected between the surrounding sedimentary rocks by volcanic activity some 335 million years ago. The island is now noted for its large colony of puffins, which can be viewed from the Scottish Seabird Centre in North Berwick. The island is a designated reserve owned by the Royal Society for the Protection of Birds (RSPB).