- Japanese: 択捉島 (Etorofu tô)
Explored by both Japanese and Russian explorers in the late 18th to early 19th centuries, Iturup was declared in the 1854 Treaty of Shimoda to be the northernmost part of Japanese territory, while Urup, the next island northwards in the chain, would be the southernmost in the Russian territory. Though Russian ships had sailed along the coast of the full island chain as early as the 1730s, it was in the 1760s that Japanese and Russians both first began to more fully explore the islands. Around 1800, the Tokugawa shogunate declared Iturup and the islands to the south to be Japanese territory, and assigned the Nanbu and Tsugaru houses to aid shogunal troops in defending the islands. These Japanese outposts came under attack in 1807 by subordinates of Russian captain Nikolai Rezanov.