Siayan is one of the Batanes Islands, part of an archipelagic province in the Philippines. Also known as Ditarem Island in the native language, the island is about 1/2 mile to 3/4 miles in diameter and lies about 5 mile north-northeastward of Itbayat Island. The closest island is Mavulis Island which is about 1.5 miles north-northeastward of Siayan. There are several detached rocks off Siayan's northeast side, rendering the channel between it and Mabudis unsafe for larger vessels. Siayan has an elevation of 538 ft and is listed as an inactive volcano by the Philippine Institute of Volcanology and Seismology.