At the northeast corner of the island is a large red-and-white daymark. It was erected in 1683 by Thomas Ekins, first steward of the Godophin Family to live on the islands.
It is a rendered granite circular tower 4.8 metres (16 ft) in diameter and 6.4 metres (21 ft) high, set back to conical termination making it 11 metres (36 ft) high. The blocked arched entrance door contains an incorrect date of 1637. It was painted white until 1822 but by 1833 had been painted red, and is now painted in bands of red and white.
There are two churches on St Martin's: the Anglicanparish church of St Martin's and a Methodist chapel, both located in the Higher Town area. The Methodist chapel dates from about 1845 and is part of the Isles of Scilly Methodist Circuit.