St Martin’s Island a small island in the northeast of the bay of bengal, about 9 km south of the Cox’s Bazar-Teknaf peninsular tip and forming the southernmost part of Bangladesh. It is about 8 km west of the northwest coast of Myanmar at the mouth of the river naf. The island lies between 92°18' and 92°21'E longitudes and 20°34' and 20°39'N latitudes. The local people call it Narikel Jinjira. It is almost flat and is 3.6m above the mean sea level. The 9.66 km wide channel between the mainland and the island is much shallower than the open sea southwest of the island. There are reefs from 10-15 km to the west-northwest.

The island is 7.315 km long and is aligned NNW and SSE. Geographically, it is divided into three parts. The northern part is called Narikel Jinjira or Uttarpara and is 2,134m long and 1,402m wide. The southern part is known as Dakshinpara, which is 1,929m long, with a narrow tail of 1,890m towards the southeast, and a maximum width of 975m. A narrow central belt or Maddhyapara connects the two parts. The length and the width of this belt are about 1,524m and 518m respectively and the narrowest collar is known as Golachipa. In addition to the main island there are a number of tiny islets ranging from 100 to 500 sq m which are locally known as Chheradia or Siradia which means separated island.

There is a shallow lagoon in the middle of Uttarpara and it is connected to the sea during high tide by a narrow channel on the western coast. The topsoil of Uttarpara consists of alluvial sands mixed with shells of molluscs. There are two small dead lagoons and a stretch of marshy land in Dakshinpara. Fish landing sites, bazars and schools are present only in Uttarpara.