Porphyry Island is an island in Unorganized Thunder Bay District in northwestern Ontario, Canada. It is the last island in a chain stretching south west of the Black Bay Peninsula in Lake Superior. It is located about 6 kilometres (4 mi) from , 13 kilometres (8 mi) from Sleeping Giant Provincial Park, 14 kilometres (9 mi) east of Silver Islet, Ontario, and 42 kilometres (26 mi) east of the city of Thunder Bay. The island and nature reserve take their name from the characteristic quartz and feldspar crystals, or porphyries found in the volcanic rocks.