Semisopochnoi is located near the confluence of the Aleutian Arc and Bowers ridge. It is the largest subaerial volcano of the western Aleutians while its summit is 1221m above the sea. It is bigger and higher and taking into account its base on the seafloor.
The island contains a caldera 8 km wide that formed as a result of collapse of a low-angle volcano following a large eruption.
In total, Semisopochnoi supports well over a million seabirds, most of which are least auklets or crested auklets. While the island has remained free of Norway rats, most ground-nesting bird species and most burrow-nesting seabirds were extirpated by foxes after their introduction during the 19th century. The fox populations have since been removed.